As a progressive, it’s simple.
Unify the opposition. That means do not split the vote against the Republicans, who are the plutocrats.
If the Democrat is better, no matter how bad, support them. Try to get the best Democrat we can. Fight for progressives to take over the Democratic Party. But unify the opposition.
The price is massive to Republicans winning, and there is no excuse for going third party in any race that is close (I’ll give you some slack in my state of CA as it wasn’t close).