The great majority of the GOP are members of the Swamp...they just give an appearance of representing the more conservative element within the Swamp.
Keep in mind that the Swamp is like a gang of criminals. If you join the Swamp, there are benefits. But these benefits inevitably accrue from unethical and/or illegal activities. Ergo, the Swamp has plenty of incriminating evidence on its members that can be used to keep them in line.
If you, as a senator or congressman, do not join the Swamp, you are a minor player with no committee leadership positions to speak of.
There are a few members of Congress who are not members of the Swamp but are still powerful...but we can never be quite sure who they are.
True Dat, else Mitch would not be Minority Leader.