Maybe I don’t understand the US system but I believe platforms are individual to candidates and that there are no overall party platforms that the candidates are expected to adhere to.

While the platforms are technically individual, they aren't really. They have to get approval from their bosses (donors) on what they get to run on. And the few actual good things that they run on has to be just empty rhetoric. If they get elected, they are expected to immediately forget those things.

when do they fight the lesser evil?

Thats the neat part, they don't. The cycle goes like this:

• It's election season. You can't point out that the child-killers want to kill children, or the evil people will win.
• Election season is over. The child-killers are in power, but their hands are tied because of the evil people who aren't in power. Its unrealistic to expect the child-killers to just stop helping the evil people kill children.
• It's election season. You can't point out that the child-killers have been killing children, or the evil people will win.
• Election season is over. The child-killers aren't in power. What do you expect them to do? They have to keep helping the evil people kill children.

Rinse, repeat