Can a presidential pardon be overruled? I know a presidential veto on a law (or bill, since they didn't sign it) can be overruled by a two-third vote in both houses. Is the same, or a similar process, available to overturn a pardon?
No (with one exception, see bottom), there is not currently any limits to the power of a Presidential Pardon. Article 2, Section 2 of the Constitution grants the power: The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment. (Emphasis mine) Those are the only limits placed on this power: For offenses against the United States (Federal crimes, not State crimes) Except in cases of impeachment The judiciary or legislative branches, according to the constitution, do not have any granted power to override or veto a Presidential Pardon. There have been a few controversial pardons taken on by Presidents: Pardon of Richard Nixon by Gerald Ford Caspar Weinberger, pardoned by George HW Bush against congressional opposition Marc Rich by Bill Clinton Chelsea Manning by Barack Obama Just before Clinton left office for Obama, he granted over 100 pardons including one to his half-brother Roger Clinton Jr.. Andrew Johnson pardoned almost all Confederate's for crimes against the United States, which was very controversial at the time. Additionally this power (granted under State Constitutions) also extends to State-Level crimes for Governors of the respective states where this power is granted. These also do not have limits and have been used quite often. The only power to override the Presidential Pardon power comes from the to-be-pardoned person themselves. The Supreme Court ruled: "A pardon is a deed, to the validity of which delivery is essential, and delivery is not complete without acceptance. It may then be rejected by the person to whom it is tendered; and if it is rejected, we have discovered no power in this court to force it upon him." This could probably be extended to State Pardons as well.
If the Senate took the extreme measure of attempting to abolish the Supreme Court, there's nothing in the Constitution that requires them to confirm any of the President's nominees. However, attempting to dismantle one of the three branches would be nothing short of a coup d'etat. The president has some options here: He [the President] may, on extraordinary Occasions, convene both Houses [of Congress], or either of them, and in Case of Disagreement between them, with Respect to the Time of Adjournment, he may adjourn them to such Time as he shall think proper. -- Article 2, Sec. 3 Basically, this means that, in an extraordinary circumstance (total defiance of the entire Senate would easily qualify), the President can lock the Senate in their chamber until they straighten their heads out. And when I say "lock them in", I don't mean that figuratively. The Constitution requires that a majority of each house be present to constitute a quorum to do business. The President could order up to 51 Senators to convene, and if they refused, could order the FBI or whoever to physically arrest them and drag them to the capitol building. This was actually done in Wisconsin by Gov. Scott Walker in 2011 when 14 Democrat Senators fled the state to avoid the quorum needed to vote on a controversial bill that they couldn't block. It would be messy for sure, and as far as I know, has never been done with the U.S. Senate. But there is precedent for it. Additionally, the President could use his vacancy power and just appoint a justice. Legally, he would have to wait for the Senate to be in recess, and the Senate would likely arrange things so that never happens. However, if there were no justices left, he could just appoint a Chief Justice (the Constitution provides that there be at least one justice). Even if he did this in the most "in-your-face" illegal way, with both middle fingers extended, the only judge that can tell him he did wrong is the guy he just seated. Of course, the Senate would move to impeach that judge, and probably the President too for having appointed him illegally, but you need a supermajority of both houses to remove them from office. If you had such a supermajority, then the game is over at this point; there's no way the President would step down in such a situation (who would force him?) and we're in a civil war because we don't have a functional government anymore. In reality, this isn't an "interesting twist in constitutional law that the Founding Fathers may not have considered." It's that if Congress ever went totally off the rails like that, the President, or the people would have to step up and hope they could end the standoff peacefully without resorting to violence, because this is the kind of thing that revolutions are fought over.
The simple legal answer is that the President has this power because the law gives it to them. Historically, the right of the head of state to grant pardons is a legacy of autocratic monarchical rule: this was a power that kings and queens had over their subjects. In France, this power ended with the monarchy during the French revolution but was reinstated by Napoleon, himself an autocratic monarch, and it has remained in the various incarnations of the French legal system ever since. Here is an examination of the concept in various jurisdictions. The concept of separation of powers is a much more recent concept. In particular, the tripartite separation into administrative, judicial and legislative power can be traced to Montesquieu in his The Spirit of the Laws in 1748. This Enlightenment point of view crept into many political systems, however, the specific expression of it reached full flower in the establishment of the United States. It is worth noting, that pardons at both state and Federal level are part of US law vesting in the Governor (or some other member of the executive) and the President respectively. The tripartite separation is supposed to provide checks and balances with each arm of government serving to limit the power of the other two. In this context, it is perfectly reconcilable that the executive branch should have the power to intervene and correct "mistakes" made by the judiciary just as the legislative branch does by being able to change the law in the light of judicial interpretation of existing law. In effect, the executive can correct "mistakes" in individual cases looking backwards and the legislature can correct "mistakes" in general going forward (the do in fact have the power to make laws retrospective [backwards looking] and specific but they tend not to do so).
No Section 2, Clause 1 says: ... and he shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment. Impeachment is explicitly carved out from the President's power to pardon.
I will only address this part of the question: Who would be able to authoritatively decide the constitutionality of such a question, with all Supreme Court justices having clear conflict of interest on the matter? The Supreme Court could still hear such a case, as the justices make their own decisions about when to recuse themselves. In particular, they might decide to hear the case based on the "Rule of Necessity", which says roughly that a biased judge is better than none at all: a judge can hear a case, even in the presence of a conflict of interest, if there is no other way for it to be heard. See United States v. Will, 449 U.S. 200 (1980), in which the Supreme Court ruled 8-0 that federal courts could try a case related to the salaries of federal judges. Another possibility is that the case could be brought in a lower federal court, say District Court. There is a question here: the Supreme Court has original jurisdiction in "all Cases affecting Ambassadors, other public Ministers and Consuls" (US Constitution, Article III, Section 2), and I do not know whether Supreme Court justices are "public Ministers". However, if a lower court did have jurisdiction, it could rule on the constitutionality of the question, since a District Court judge would not have a significant conflict of interest. The relevant Circuit Court of Appeals could presumably hear an appeal. If the Circuit Court's ruling was appealed to the Supreme Court, and the Supreme Court felt that they all had conflicts of interest (and decided not to invoke the Rule of Necessity), then they could simply not vote to grant certiorari, in which case the Circuit Court's ruling would stand.
If the Senate has convicted by the necessary majority, Trump would no longer be the President - Mike Pence would be. There is no legal difficulty in removing ordinary citizens from the White House. Of course, President Pence might decide that he didn't mind ex-President Trump remaining in the White House, and I think the law enforcement agencies would be obliged to respect that.
Yes, the President can just fire them, or more commonly, request their resignation. The provision for impeachment by Congress exists in case the President doesn’t do what the President should do.
Could the acceptance of the pardon then have any bearing on the case in the other jurisdiction? Possibly, but not much. There is very, very little case law on this point since: (1) pardons are rare (especially federal ones), (2) people who are pardoned generally do so because everyone in the criminal justice process in the prior case agrees that the person is reformed and they are usually correct, (3) the statute of limitations has often run on a new prosecution, and (4) many cases where these issues arise, are probably not appealed (either because the neither parties attempts to, or because a defendant is acquitted and there is no appeal), but an appeal is necessary to give rise to binding precedents. Can the admission of guilt be used in the new case? The criminal collateral estoppel effects of a pardon flow from the adjudication on the merits which is vacated. Also, comity between sovereigns and public policy mitigate such a rule. The pardon power would not be very interesting if it routinely resulted in a new prosecution that was conducted on a summary basis via a preclusion doctrine such as collateral estoppel. In many cases, the statute of limitations will have run on the original crime or there will be no parallel state law crime, but this is not always the case. Also, I would disagree with the statement that a pardon always implies an admission of guilt to the crime for which a pardon was granted, even though that statement is often used rhetorically. For example, one important use of the pardon power is to commute the sentence of someone who asserts that they are factually innocent but have been convicted of a crime, potentially in a manner that is not subject to further judicial review, and treating that as an admission of guilt doesn't make sense. As the Wikipedia entry on Burdick notes in the pertinent part: Legal scholars have questioned whether that portion of Burdick [ed. about admission of guilt] is meaningful or merely dicta. President Ford made reference to the Burdick decision in his post-pardon written statement furnished to the Judiciary Committee of the United States House of Representatives on October 17, 1974. However, said reference related only to the portion of Burdick that supported the proposition that the Constitution does not limit the pardon power to cases of convicted offenders or even indicted offenders. I would read this as dicta, as this portion of the holding was not necessary for the court to reach its conclusion and the fact pattern in Burdick was a typical fact pattern where guilt was not disputed. It didn't raise the concerns present when a pardon is requested based upon a claim of innocence, and granted following a conviction. Instead, the holding of Burdick was that there was no pardon because the pardon was rejected (in a manner very similar to a common law disclaimer of a gift), so its holding didn't need to reach the effect of a pardon that is accepted to resolve the case. Can it be used as &quot;reasonable cause&quot; for various actions? This is a bit too vague to know what you are getting at. I suppose that a pardon could constitute reasonable cause for some things favorable to a defendant who is pardoned (e.g., potentially in a motion seeking to reopen a termination of parental rights entered on the basis of the conviction). I suppose it could also be used in a manner potentially unfavorable to a defendant (e.g. showing a pattern of past conduct that demonstrates modus operandi in connection with a prosecution for a new crime). I don't think it could be used as grounds to deny an occupational or business license for bad character. Still, without more clear context it is harder to know what you are really looking for in this regard and I'm not confident that my examples address that. Does the defendant lose their right to refuse to testify in the new case? I haven't reviewed the case law, but my intuition is that if it has never been waived before, it wouldn't be waived by the pardon, but that if it was waived in a previous proceeding resulting in a conviction that was then pardoned, that the prior sworn testimony might be admissible evidence in the new action since it is not hearsay and isn't itself evidence of a prior conviction. The context of the prior testimony might have to be concealed from the jury. As noted by @Putvi, the defendant could not claim risk of conviction for the federal crime as a ground for invoking the 5th Amendment if a pardon is accepted (something that is implied in Burdick), but if there was an overlapping state law crime, risk of conviction for the state crime could constitute a grounds upon which to invoke the 5th Amendment. Burdick does stand for the proposition that a pardon not solicited by the defendant, that is rejected, cannot provide a basis for removing the 5th Amendment protection with respect to a risk of conviction for federal crime. I would also be inclined to think that matters disclosed in an application for a pardon might be admissible evidence as a non-hearsay statement of a party-opponent, if the statement was stripped of the pardon application context (which would be unduly prejudicial since it would imply a prior conviction which otherwise wouldn't be admissible).