The two big parties are coalitions of groups held together by hating the other coalition more. Ever few generations there's a "realignment" and groups switch between coalitions. FDR's New Deal was one, the Civil War was another. The Democrats are composed of blacks, Hispanics, union members, and liberals. The Republicans groups are rural, Christian, and business. This is of course an approximation. Every one of those groups breaks down fractally into thousands of sub-cultures, and other little fragments adhere to one side or the other. But these are the groups large enough to show up in opinion polls.
The Republican primary was supposed to be dominated by candidates locking up each of the groups and then having a run-off among the final two or three ("lanes" in the consultant jargon). Trump instead split each group on class lines. He's bringing in the people who are doing badly in each group, the ones whose death rate is increasing. For example, the Christians split between Trump and Cruz. If an evangelical Republican reported attending church more than once a month, they were a Cruz voter. Trump got the "evangelicals" who didn't go to church--and presumably lacked other social support networks. Likewise the rural conservatives whose towns were dying and small businessmen afraid that they're one OSHA ruling away from bankruptcy.
The Democratic coalition is also vulnerable to splits. The unions are separating into government workers and industrial ones, with the non-government workers being laid off or watching their companies go bankrupt. More than one primary had Trump boosted by those Democrats crossing over to vote for him. They'll probably stick with him in the general.
Trump is endorsed by some black celebrities. He's in a position to drive a wedge between blacks and Hispanics. There's already tension between the groups because they're competing for overlapping jobs and housing, generally papered over in elections by pointing to racists as haters who can't tell the difference between the groups. If Trump pushes immigration restrictions as a way to increase the economic fortunes of blacks he could pull some to his side. I wouldn't expect a majority, but if 10-20% vote for him, and a 10-20% stay home because they're not afraid of him, it could tip some states.
Liberals don't seem like a good group for Trump to look for votes in, but again there's class divisions within them. The Occupy Wall Street protestors were as liberal as tenured professors, but they were marginally employed (if at all) and crushed by student loan debt. Those are people who hate bankers more than they hate Trump. Anyone who can quote how much Clinton collected in speaking fees from Goldman Sachs is a potential Trump voter. He can point to his bankruptcies as a blow against the bankers. A proposal to make student loans dischargable in bankruptcy could bring 10% of the liberal bloc to his side, enough to tip a couple states.
Will this be enough to outweigh the Republicans who are sticking to #neverTrump by going Libertarian or dropping out? Beats the heck out of me. But I think there's a plausible scenario that Trump could be our next president. Horrifying, but the Republic will survive.
The more interesting question to me is whether the party realignment will revert to old pattern or keep changing until we have two new coalitions.
(If you want another scenario for how Trump can win, check out Scott Adams' persuasion series at the Dilbert blog)
Me, I'm planning to vote for Gary Johnson, whether the Libertarians nominate him or not. He would only have to take a few states to throw the election into the House of Representatives, which might prefer him to the other two.