The must. What are the options?
Option 1 - force a withdrawal. Elect a Democrat.
Option 2 - run a write-in. Elect a Democrat.
Option 3 - support the candidate. Elect him. Swear him in. If the charges are substantiated, he resigns and the Pubbie governor appoints a replacement.
Those 3 options are the only options and number 3 is the way to go.