I have wondered about that myself. They are, however, 50% of the congress (or very close to that). I don't think that one side can remove the other! There are also rules (I think, have no idea what they might be though).

That being said, those knowingly supporting The Big Lie, I think, can't possibly be allowable as the big lie is the antithesis of this nation's constitution, I think (election denial). Basically, if you support the big lie then you are a traitor to the nation.