any more than openly admitting he'd collude with a foreign power
This actually does the opposite of hurt him. I mean, they'd ALL collude if it meant getting dirt on their opponents, it's just the vast majority of politicians would lie about it.

sexual assault allegations
I allege you have sexually assaulted me.

You're up to one, you disgusting human being.

That's how easy it is, if you're holding "allegations" against people.

So far, he has one black mark against him as far as I'm concerned, and that's the treatment of migrant detainees. That would possibly be enough to stop me voting for him, if I had the ability to vote in a US election, but that really does depend on who he's up against. If he's up against, say, Hillary Clinton, I'd weigh up the pros and cons and I'd probably decide that a few ill treated migrants is a lesser evil than creating migrants.