I find the idea that they are the same to be a wilfully obtuse deflection employed by centrists to avoid confronting the catastrophic foulness of the Democratic party.
The left criticism of the Democratic party is not, and never has been, the claim that both parties are exactly the same.
But it is entirely possible for two things to be (a) not identical, and (b) both catastrophically bad.
America has two parties, both of which are corrupt, murderous, plutocratic and racist. One of them is subtle and devious, the other is blatant and crude.
Neither of them are forces for good in the world. One party offers a slightly slower and more polished catastrophe than the other, but neither party offers a path that is consistent with justice or long-term survival.
There are harm minimisation arguments in favour of voting for the slower evil, although those arguments are not themselves unassailable. There are also arguments in favour of continuing attempts at reform/subversion within the existing party structures. But those arguments do not imply that it is a wise course to deny the reality of the situation.
Most of the left has not yet abandoned electoralism entirely. But very many people are justifiably on the verge of doing so, and half of the country was already there long before Trump arrived.
Fortunately, electoralism is not the only means of pursuing political change.
