I don't see how the race of Trump's base is relevant. The United States is a majority white country. Hillary's base is predominantly white, ditto Sanders and Cruz. Also, Trump polls far better than his republican opponents amongst blacks and latinos. But yes, if he were to go up against Clinton he'd likely lose, however, there's a lot of time between then and now, and nothing is ever certain in politics. If he were to go up against Sanders, however, that's anyone's guess. The US is strongly anti-socialist, for good reason in my personal opinion.
Overall though, my guess is that Trump is being bombastic and outrageous now for all that free media. If he wins the candidacy he'll start becoming a lot more moderate. Ultimately though, I think a lot of the anti-muslim, anti-immigration talk is a gamble. He's banking on another major terrorist attack or two in the west before the presidential vote. If that were to happen, no one would be in a better position than him to win over new voters on that issue alone.