Well, I would argue that that is like 95% where their votes are coming from, basically "This is still the 'left' option, I guess," rather than believing in any sort of positive vision on the part of the Democratic Party (it doesn't have one).

However, politics isn't just a 1-dimensional spectrum where things neatly slot into whatever is closest. The fact that they are lurching rightward, the apparent contempt they have for the left, the lack of any meaningful similarity between what a left-wing person wants and what the Democrats will even acknowledge is real (like action on the genocide in Palestine), means that what you are taking as similarity is in many cases difference. Just saying "Fuck you, vote for me because the other guy is worse" is really not a good strategy for getting votes unless you are holding getting votes as secondary to pandering to donors.

Like, do you think a new Republican candidate could just be blatantly pro-choice and not lose one or two dedicated blocs of the Republican voting base, just because "he's still the farthest right"? Of course not, democracy doesn't work that way. If you don't support people on the issues they care about most, a good number of them will tell you to go to hell while the others roll over as always.