The "leadership" of the Democrat party sits to the right of several disparate further left factions. Because they don't embrace any specific leftward direction, they are juggling half baked compromises instead of leading anywhere. Bold policies that would be approved of by one further left group are opposed by others, so they can't go left without losing support somewhere. Staying where they are makes them moderately disagreeable for every one of the factions that can support and vote for them, so they are unpopular across the board. They are all but trapped not far enough to the right to contend for Republican votes, and not far enough left to propose anything truly different.
I see the candidacy of further left individuals (mostly at the local level for now, but this will move fast if the "leadership" collapses further) as the first serious mechanism to break this stalemate. Popular figures from city or state government aim for national positions frequently, so expect anyone standing out with how well they run things at the local level to make that pivot.
A similar thing is at play on the right. Christian fundamentalists, war hawk neoconservatives, the alt right, the would-be fascists, select business interests, nationalists, libertarians, and others are constantly battling over policy. At this moment about 70% of them are trying to ride Trump's popularity and apparent effectiveness at making changes to get whatever is most important to them done before it's too late. If the Republicans gain seats in 2026, that surface level unity will become even more significant, but once Trump is out of the picture, infighting is all but certain to resume on the right, and we'll see weaker, "keep everyone happy" politicians take center stage again.
If both those processes play out with the right timing, we may get a true leftist running against a Jeb tier Republican in 2028 or 2032.