Question

Who do the Democrats prefer? Whom do the Democrats prefer? Which sentence is correct ?

Answer 1

Whom.

Turn the sentence around and make it a statement.

The Democrats prefer whom. It's clear that whom is the object of the verb "prefer."

Answer 2

Whom can you believe? You can believe whom
You would use whom because it is an object of the verb. Is this correct?

Answer 3

Yes, "whom" is correct in that question.