One of two things will happen:
1. They will "punish" Menedez by "censuring" him (just as they did with Charlie Rangle), he'll keep his senate seat, and that will be the end of that. (this is the most likely scenario)
2. If Menedez is forced to resign, the Dems will insist that Chris Christie name another Democrat to replace him, and Christie will give in to that demand.
There is speculation that Cory Booker will be the replacement for Menendez. Obama wants more blacks in the Senate.