Menendez was appointed to the Senate to fill Corslime's senate seat when he became Governor. Menendez was an elected congress-critter prior to the appointment.
I'm just pointing out that it would be difficult to come up with an excuse that merits pulling him off the ticket since he is a sitting Senator right this very moment in Washington. Not that they couldn't make something up, but they'd have to somehow remove Menendez from office for the remaining 3 months of the current term to do this switcheroo...