... if he's elected.
Here's an article about that.
It offers three possibilities. One is to refuse to seat him, but it says that can only be done because of the irregularity of an election, e.g. ballot-stuffing or bribing.
The second is the extraordinarily high bar of actually expelling him, something that hasn't been done since Confederate sympathizers in the Civil War.
But the third (no. 2 on the list) is to refer his case to the Ethics Committee. The article doesn't say this, but it could defer his seating until the Ethics Committee had made his report. Something similar to this has happened since the Civil War. The notorious Mississippi racist Theodore G. Bilbo was investigated for his 1946 re-election on grounds of his inflammatory campaign tactics and shady finances. But impasse over whether to take action was postponed when Bilbo became ill and did not insist on being sworn in until he'd recovered and returned to Washington. The committee reports were tabled (which in US discourse means action was deferred). But instead of returning, Bilbo grew more ill and died several months later, which rendered the issue moot. Here's the official version of that story.