I actually think shit like this does backfire on him, and hurts him politically more than it helps. Until his first tweets and his doubling-down on them, the House Democrats were fighting among themselves and looked to be a mess. With this blatantly racist and xenophobic attack he's managed to unite nearly all Democrats again, and while his base no doubt loves the tweets, he's also enraged the opposition. This is why I think next year's election will be close no matter who the Democrats nominate. Trump will never win anything by a comfortable margin, much less a landslide. Reagan was a charmer who could win over a large majority, Trump by nature is a divider and hater who drives away at least many people as he attracts. He may well win next year, but if he does it will be by a narrow margin, and another possible popular vote loss (with a slender EC victory) is a real possibility.