I'm not 100% certain, but believe they can per Section 5 of the U.S. Constitution:
Each House shall be the Judge of the Elections, Returns and Qualifications of its own Members,...Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member.
If they do seat him, the Dems would eagerly join in expelling him. So two-thirds should be easy to achieve. But I think the first clause about each House being the judge of the elections and qualifications of their own members could be used to refused to seat him.
No matter what happens, it's a poisonous situation that can have no good outcome. For way too long, we have allowed peoples lives to be destroyed on mere accusations without proof or trials. Accusations of racism, sexism, sexual harassment, and all the other Leftist identity categories. Yes, it's recently come back to bite some Hollywood figures, much to our satisfaction, but most of the time these types of accusations only are used against and destroy Republicans. So, no matter who the target is, I always remain skeptical until and unless proof surfaces.
I did not mean they couldn’t do it legally. I meant only that I don’t think there would be enough voting against him to get the necessary vote. I don’t think they’d have the guts to tell Alabama voters they can’t elect the senator of their choice. Particularly in this age of blatant election fixing.