If he was convicted, and he didn't have the honor to step down, the Senate would vote to kick him out at that point. The Senate has routinely voted to kick people out convicted of serious felonies. And he would be replaced with another Democrat anyway. Plus the establishment dems would be happy to see him go.
If he's convicted, and if he doesn't step down, he would be kicked out of the Senate by a vote of 99-1.