First, extreme party politics works for the right because they lack diversity in all aspects so it's easier to push a direction without destroying support. Its the opposite for the dems because what makes the party so appealing also makes it harder to direct.
So engage on this for a second
Extreme party politics works for Republicans better than it does for Democrats
You want the parties to come together and compromise on issues instead of just fighting for their political platforms all-or-nothing.
What happens if Democrats put forward candidates who compromise as a first action against a party of rabid "Our way or the highway" types? What do we get done?
The same thing that happened post 2012 for Obama, nothing. Nothing gets done because compromise is dead, and as a minority party the GOP will only support their own legislation. The Republican party has,
as an actual rule, that they cannot and will not support any legislation that does not receive 50%+ support internally. They have
an actual rule against compromise, and you want a compromise candidate.