Trump is a controversial leader who promoted, all the way through, opposite ideas. Consistency is mandatory to win an election and doing not ruin chances. Ann Coulter's launch book would have been a success for trump if this one had better defended his immigration policies, providing evidence that his plan will prevail in the future. None of that happened.
Donald Trump has had the Russians helping him with their hacking and he still can't overcome his big fat mouth. Every time he opens that mouth it is always wide enough to stick his foot in it. His most recent mess up is his flip flop on immigration. There is no way he will get elected when he says one thing one minute and another thing the next minute.
Trump stands no chance of winning the presidency as it is, and this is only going to make it worse for him. He has a smaller and smaller number of supporters each day it seems, and changing his stances that his supporters actually agree with is only going to make his base even smaller.
For all intents and purposes, this presidential race has been long over. Trump simply doesn't have the ability to attract enough voters in a general election when more of the country is paying attention. If he flip-flops now he will rightly seen to be just pandering without conviction, and he'll lose a large chunk of the supporters he's already gotten. He's painted himself into quite the corner, and this latest debacle is just one in a long string of them.