This just simply isn't true. From what I've observed Trump has the most self-critical "movement" of any recent president. They spat in his face over a mostly symbolic Syria strike and he's experiencing heavy backlash over DACA. The front page of Breitbart is all anti-Trump right now. It isn't that they're fickle -- it's that he was elected explicitly on the condition of two or three policy promises and he is merely an instrument.
What did happen in both cases is that mainstream/liberal Republicans suddenly began praising him and since those of you on the left have ceased to differentiate between anyone outside of the Democrat party, you mistook it for his base flip-flopping. Either you are unperceptive or lying.
There is one piece of truth to what you're saying, though -- Trump could flip-flop way to the left of his current centrist platform and he wouldn't effectively lose many votes for reelection because all of the alternatives in US politics are even further left.
All of the "crazy right wing" things he's attempting to do? Revert Obama policies back to Clinton policies. America has no actual right-wing by global standards, therefore the candidate who is least left-leaning will always get our votes. It is our weakness, but we are vocally critical.