How does a 3P POTUS advance any of their agenda through a Congress where their party controls exactly 0 seats?
The 3P POTUS candidate obviously doesn't have down-ballot support and won't until there's actually a viable 3rd party.
Then that 3rd party has to be ideologically situated somewhere, either further right than the Republicans, further left than the Democrats,
or, in the most pathetically useless case, right in between them.
People are making a protest vote, not expecting that candidate to win anything,
because they perceive no downside to doing so. They believe that whichever major party candidate actually wins,
and they know one of them will,
one will be just as bad as the other would have been.
Three viable parties, by the way, will essentially mean the end of presidential elections.
Meaningless ones could be held, but the House of Representatives would actually choose all further presidents.
A 3P candidate makes sense for Oom because a Libertarian with whom he somewhat agrees will likely be on the ballot.
A 3P candidate makes no sense for me because no candidate will match my requirements, and the closest one would be
the actual Democratic nominee.
I am a social conservative [by modern Democratic standards, obviously]
and a fiscal socialist. I am an old school FDR Democrat.
Biden isn't close to what I'd like,
but unfortunately, he'll be the closest thing, despite not being very close.