In one of my cheating campaigns (the best cheats are the manipulation of character traits, ancillaries, and buildings files...I don't do that normally, though, it'll only ruin the game in the long run) I assassinated all Senate family members (the trick is not a super-assassin one, but a super-vulnerable trait/ancillary for targeted characters one). No senate member alive in sight - and I had spies and assassins all over Italy by then. They did not fall...

Hmm...