system
Given the following post, respond as if you're commenting on Reddit:
human
The Green and Libertarian Parties are a waste, and people who want to advance those causes should work within the Republican and Democratic parties instead.

The Green and Libertarian parties have completely failed to elect any significant officials, not presidents, Senators, governors, major city mayors, or even Congresspeople.

They don't get their issues on the agenda in any meaningful way.  The mainstream media just ignores them.

The US' winner-take-all political system basically makes third parties futile.  In many countries in Europe, if you get X% of the vote, you get X% of the seats in parliament, and that system enables alternative parties to have a voice and to have representation.  In the US, you have to win your state or your district outright or you get nothing.  A party with 10% support in every district nationwide wouldn't get anything for that, and the Greens and Libertarians aren't even close to that.

The Christian Right, which represents at most 10% of the country, works within the Republican party, and has unfortunately been very effective at getting its agenda enacted...while libertarians and their separate party have been totally ignored by mainstream Republicans.  It seems clear to me which strategy is more effective.
gpt
The strength third parties have is threefold:

1. They can bring new issues to the political table.

2. They can draw enough votes away from main parties to affect elections.

3. They can make a big enough deal about certain issues that the main parties pick them up.

Historically, parties have had effects on both the political system and the laws it made:

The Anti-Masonic party popularized nomination conventions and party platforms, which both the Republican Party and Democratic Party now do.

The Prohibition Party got the 18th Amendment passed, prohibiting alcohol, which was a HUGE deal at the time.

The Progressive Party helped popularize the ideas of recall, referendum, and initiative in regards to politics.  They also brought attention to a lot of tax laws, social insurance, workman's comp, and woman's suffrage.  Their candidate (Teddy Roosevelt) also got 27% of the vote in the 1912 election, which is pretty impressive for a third party.

The Libertarian and Green parties have raised tons of awareness for their ideas, as well.