They don’t believe that the party is innocent at all. They know that awful shit is happening, but they chose to lie to themselves, and others, because the Republicans deliver something that they value more than their self respect, or respect of others. For most, it is permission to hate. There’s someone, or some group that they hate so much that they are fine with whatever is “actually happening” as long as they can hate, and their hate is rightous and justified. With enough little lies like “both sides are doing it, so what are you going to do?” stacked on top of each other, then they can disassociate the awfulness from themselves and their party, and shift their disgust for what is being done to the other party.
You just have to realize that the strength of their hate for the Democrats is actually how much they hate then and their own party. So, knowing how much mental effort and emotion goes into lieing to themselves and everyone else, you now have some idea of how truly awful their little secret is.