I don't trust most career politicians.... I don't care if they have an "R" after their name or not.
Trump's an outsider, and as such he's hated/feared by everyone in the establishment... most of those people have a lot to lose if he succeeds in breaking the corrupt system.
Those career politicians in Congress make or break the agenda.
If R’s start defecting on issues like this, it makes it harder to keep them in line when its time to vote on things, like repealing Obamacare