Who becomes President if no candidate for President or for Vice-President has been selected at noon, January 20th? The Twentieth Amendment to the Constitution , Article 3, deals with the situation where no President or Vice President has qualified on January 20. Section 3 If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified. (Emphasis added) So, has Congress made such a provision, and, if so, what is that provision?
If no President or Vice-President is picked by Inauguration Day, January 20th, then the Presidential Succession Act kicks in. The Act lists the line of succession Acting President. It starts with the Speaker of the House, the President pro tempore and then goes through the cabinet officers. (You can see the full order of succession here.) You can read more here here and here.
Yes, it has happened. Charles Evans Hughes served on the Supreme Court from 1910 until 1916, when he resigned to run for President as the Republican nominee. He lost to Woodrow Wilson. Later, in 1930, he was appointed again to the Supreme Court, this time as Chief Justice, and served there until 1941. John Jay, Oliver Ellsworth, and James Iredell all received electoral votes for President in the election of 1796. Jay had recently resigned as Chief Justice in 1795, and Ellsworth and Iredell were still serving and did not leave the court until 1799 and 1800 respectively. It's not clear that any of the three seriously campaigned for the presidency, though.
Most of the implications here are political, not legal. For that, you'd have to ask Politics.SE. The law, however, is quite clear: If the President is alive, and &quot;a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide&quot; do not invoke the 25th amendment, the President would remain President and just not do their job in this case. Congress could presumably provide for a different set of people to validate the President's disability status, but in your hypothetical, this wouldn't happen due to a lack of consent from the Senate. The 25th amendment was created to solve this problem: it is the only solution to it. If it is not used, then there is no other means to remove a disabled-but-alive President. Most (all?) executive agencies can run themselves perfectly well day-to-day without the President's help, so nothing would be likely to fall apart immediately. There wouldn't be anyone to appoint new judges or other presidentially appointed officers, which would probably eventually become a problem.
This is an interesting question. Requirement to &quot;Qualify&quot; The relevant provision seems to be subsection (b) of the Presidential Succession Act of 1947 (codified at 3 USC 19. That subsection provides that: (b) If, at the time when under subsection (a) of this section a Speaker is to begin the discharge of the powers and duties of the office of President, there is no Speaker, or the Speaker fails to qualify as Acting President, then the President pro tempore of the Senate shall, upon his resignation as President pro tempore and as Senator, act as President. Thus a Speaker would not become acting President unless the Speaker &quot;qualifies&quot;. I believe this would include fulfilling all the requirements for a President, including the minimum age, and the requirement that the President be a &quot;natural born citizen&quot;. Subsection (d)(1) further provides that: If, by reason of death, resignation, removal from office, inability, or failure to qualify, there is no President pro tempore to act as President under subsection (b) of this section, then the officer of the United States who is highest on the following list, and who is not under disability to discharge the powers and duties of the office of President shall act as President: Secretary of State, Secretary of the Treasury, Secretary of Defense, Attorney General, Secretary of the Interior, Secretary of Agriculture, Secretary of Commerce, Secretary of Labor, Secretary of Health and Human Services, Secretary of Housing and Urban Development, Secretary of Transportation, Secretary of Energy, Secretary of Education, Secretary of Veterans Affairs, Secretary of Homeland Security. The Act has never been invoked, nor have either of the two prior Presidential Succession Acts, so there is no case law or precedent available. The Wikipedia article on the Act describes the history of all three versions, and the times when an invocation might have occurred. Questioned Constitutionality The Wikipedia article quotes a statement by Judge M. Miller Baker made during a September 2003 joint hearing before the U.S. Senate's Committee on Rules and Administration and Committee on the Judiciary. This statement argues that the Act as currently written is unconstitutional. The statement reads: The 1947 Act is probably unconstitutional because it appears that the Speaker of the House and the President pro tempore of the Senate are not &quot;Officers&quot; eligible to act as President within the meaning of the Succession Clause. This is because in referring to an &quot;Officer&quot;, the Succession Clause, taken in its context in Section 1 of Article II, probably refers to an &quot;Officer of the United States&quot;, a term of art under the Constitution, rather than any officer, which would include legislative and state officers referred to in the Constitution (e.g., the reference to state militia officers found in Article I, Section 8). In the very next section of Article II, the President is empowered to &quot;require the Opinion, in writing, of the principal Officer in each of the executive Departments&quot; and to appoint, by and with the advice and consent of the Senate, &quot;Officers of the United States&quot;. These are the &quot;Officers&quot; to whom the Succession Clause probably refers. This contextual reading is confirmed by Madison's notes from the Constitutional Convention, which reveal that the Convention's Committee of Style, which had no authority to make substantive changes, substituted &quot;Officer&quot; in the Succession Clause in place of &quot;Officer of the United States&quot;, probably because the Committee considered the full phrase redundant.[1] Since the Act has never been invoked, this contention has never been tested in a court. Given the 25th Amendment's provisions for filling a vacancy in the office of Vice-President, the likelihood of the act ever being invoked is much reduced. Notes [1]: &quot;Ensuring the Continuity of the United States Government: The Presidency&quot;. Prepared Statement of M. Miller Baker, Joint Hearing Before the Committee on Rules and Administration and the Committee on the Judiciary, United States Senate. September 16, 2003. Archived from the original on January 14, 2021. Retrieved July 11, 2018 – via GlobalSecurity.org. [This citation is copied from the Wikipedia article.]
The comments have basically covered this, but: It's a slightly weird parallel structure ("who shall not be at least 25, and been a citizen for 7 years, and who shall not be a resident"). The Constitution is not written in fluid 21st-century English. But the obviously correct way to parse the sentence is that no one can be a representative who isn't a 25-year-old or older who's been a citizen for at least 7 years; furthermore, no one can be a representative who wasn't a resident of the state they represent when they were elected. With some parts of the Constitution (like the Second Amendment), the drafting results in actual disputes about the intended meaning. With other parts (like here), only one reading makes any sense. It's the same with the requirements to be President. The Constitution says "No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President," which could be read as "you're not eligible unless, when the Constitution was adopted, you were either a citizen of the US or a natural-born citizen." But that's a silly reading, so "at the time of the Adoption" is read as only applying to "a Citizen of the United States:" natural-born citizens are eligible period, and people who were born before the US was a thing but were citizens by the time the Constitution was adopted were grandfathered in.
Before the election, can a lawsuit prevent the amendment question from being put on the ballot? No. Generally speaking, the constitutionality of an otherwise procedurally proper ballot initiative is not ruled upon until after it has passed (eliminating the need to rule unnecessarily on the constitutionality of failed initiatives). Of course, if someone tried to put it in the ballot when the existing law conditions for putting it on the ballot were not met (e.g. because a deadline for doing so wasn't met) that could be challenged in court by a party with standing to do so. If passed into the constitution, would a court still have the ability to rule the restrictions unconstitutional? Probably not. The constitution as newly adopted doesn't sound like it would forbid doing that. This seems to be the whole point of the constitutional amendment in the first place and it the amendment to the constition is approved, that requirement is gone.
Assuming, for purposes of argument, that such a change could be made by executive order, or indeed by legislation, rather than by amendment, yes, the change could and would have to have an effective date, and apply only to persons born after that date. Indeed that would still be true if such a change were made by constitutional amendment. Consider, people whose parents (or one of whose parents) are US citizens, but who are born outside of the US, are or may become citizens (by statute), but only if the citizen parent complies with certain rules specifying a minimum period of residence in the US, and other conditions. Those periods and conditions have changed over the years, and each such change had an effective date. If such a change could be made but could not be made with an effective date, then it would apply retroactively, and deprive people who are already citizens of their citizenship. There is no valid equal protection claim, because the same rules apply to both individuals, they merely give different results for the different birth dates. (See my answer to this question for why I do not think such a change would be valid except by Constitutional Amendment.) EDIT: I suppose such a changed interpretation, if made at all, might take the position that such births never did validly convey citizen ship, and all people whose parents were not validly in the country had never been citizens. That strikes me as even less likely to pass judicial review, but one cannot know what the Supreme Court will do when faced with a truly novel situation.
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.