In a way, it can be argued that collectively we get what we deserve. This is the problem of democracy--it relies on the quality of the people involved, leaders and citizens. No, I am not advocating authoritarianism; I'm merely pointing out the obvious, which has occurred many, many times in the past. Indeed, if Trump didn't have many millions of Americans behind him, he would have been in the lunatic fringe.
Though, another argument can also be made that the lunatic fringe has been petted, entertained, and even cultivated by the Republican party in the last 30 years. The Trump phenomenon is not new; it just happens that an arrogant egomaniac has a megaphone through which he expresses what a significant size of Republicans already believe and say. Yeah, even the third-grade language (no complete sentences), insults, vulgarity, ignorance of issues, and prejudice of the ..angry base have found a loudmouth to be uttered in the political debates of the elites. As I said before, this is not good for our country and the way we should conduct our political discourse.
Now, how should the Democratic nominee deal with Trump? To begin with, he has to be taken seriously--a lesson the Dems are learning today by watching the GOP's contest unfolding. To rely on the news media to challenge Trump's inconsistencies and voodoo policies isn't enough judging from history. Ridiculous claims must be addressed; don't rely on the media or the public to dispel them.
I don't think it'd be hard to get under Drumpf's skin and then watch his go unhinged. But, his appeal should not be underestimated. He has a message. His logo--Make America Great Again--is the only one people can recite. I doubt most people know what the other candidates' logos are! He's simplistic but that's easily understood (it doesn't have to be an intellectual understanding) by anyone, especially the low-information voters.
A couple more observations. The country is moving in a progressive direction, despite the loud noises coming from the conservatives and reactionaries. The Supreme Court will soon take a more progressive path. What was radical 20 years ago, it's mainstream today. Even the world "liberal" is now adopted as one of their identifiers by the majority of Dems in all states, except Oklahoma, though it's strong there too. Back in 2008, the majority of Dems didn't want to identify themselves as liberals.
This election, like many others, will be decided by turnout. The more people vote the better for the Dems. We've heard that Trump has brought in millions of voters, which is probably true, but he carries very high negatives, which alienates many conservatives who may stay home on election day. Despite the low(er) numbers in the Dem primary, the groups normally supporting them will be energized next Fall, especially if Drumpf is the GOP nominee. He has alienated some of the Republican base, many of the so-called independents, the Hispanics/Latino, Asians, and women.
Here's a view from the conservative elite discussing Romney's intervention against Trump and why some Republican don't like him. Or, "why this Republican party must die".... [link, CNBC]
The Dems are more united and generally happy with either Clinton or Sanders. The SCOTUS issue is big and will loom even bigger this year given the obstructionist Congressional Republicans. The battle for the Supreme Court can energize both parties' bases, but here the Dems have an advantage in numbers, especially among women. Did you hear that single adults are now the majority of Americans? Given that women are the majority in the US, then single women are a powerful political block. Why "political"? Because they're motivated by political issues of great concern to them, like health care, education, reproductive choice, etc. They are overwhelmingly pro-choice, for example, and they care about other issues liberals/progressives champion.