The Senate could expel him, but for various reasons I'm not sure Democrats want to do that
They should want to. Aside from being, you know, the right thing to do, it would also put Republicans on the spot if Moore wins so that they have to expel him as well.
Don't be naive. Republicans have nothing to lose by not following political norms. If that was true, we wouldn't have Neil Gorsuch on the Supreme Court right now. I could come up with a few more examples if you'd like to debate this point further.