Those of old enough to remember Bush The Elder's pronouncement, "Read my lips, no new taxes", are old enough to remember what happened when he failed to deliver on the promise so that he could make nice with Democrats in the Senate. That promise had been instrumental in getting him elected. Breaking it removed any chance he would get re-elected.
Where I live, a man's word is all he has. Trump promised. If he fails to deliver, he will pay the price with people that will not trust him twice. They may not vote for a Democrat, but they will just stay home.