He talks big to cover up his political and other inadequacies hidden under his loud screechings. If he was simply just a clownish, oafish GOP anomaly, then there would be no harm done -- he'd simply fade away after his 15 minutes of fame. But the problem for the Republican Party is that as extreme as he his he's a party favorite -- the crackpots are now running the GOP. For the GOP Trump is dangerous because he understands that beating up on immigrants and the defenseless is good Republican politics -- It's the way to win the heart of the rank and file GOP voter.
Trump and people like Ted Cruz may have already re-constituted the Republican Party. Its no longer the Republican Party but a neo-rump construct -- a political organization that has sown within it the seeds of its own destruction and which now fights a losing battle against a smaller, more rabid version of itself that it encouraged because of its hatred for America's first Black President.
Today, the Republican Party is reaping what it sowed and Donald Trump and others like him is the unholy result. Today, the Republican Party's base of voters is overwhelmingly scared, white and old, cultivated by two generations of Republican officeholders who played on their resentments and fears. Out of this came the Tea Party and its darlings -- Donald Trump et al.
Republican white voters don't much like illegal immigrants and don't look kindly on politicians who want to improve the party's appeal among Hispanics and Blacks. They like Trump's racist demogery. And that's the problem for the Republican Party -- it can't dismiss Trump any more than it can disown the Tea Party.