Part of why any Democrat is automatically superior to Trump is that any Democrat would be at least partially beholden to Democratic elected representatives and voters, and would, out of expediency if for no other reason, feel pressure to respond to that base.
Trump, of course, feels no such constraint. His supporters are fanatically loyal, meaning he can basically do what he wants. When he does face base pressure, it pushes him towards far-right positions in areas he doesn't seem to care much about, like welfare policy or education.
By voting against Trump, you vote to disempower his party, too. And by voting for a Democrat, you vote to empower the Democratic policy and political infrastructure, much of which will remain the same regardless of who the nominee is.
That doesn't mean some of the Dem candidates aren't awful. (You should not, under any circumstances, vote for Mike Bloomberg in the primary.) But voting decisions aren't really about picking between abstract ideologies, they're about elevating different political coalitions.
"Vote blue no matter who" gets mocked on here as a wishy-washy liberal slogan, but the reality is the opposite: party-line machine voting is an unsentimental way of winning power, which is why it's often wielded by groups who don't have the luxury of not caring who's in charge.
