So how will he win? "I think I'm going to go out and I'm going to get millions of people from the Democrats," Trump said. "I'm going to get Bernie [Sanders] people to vote, because they like me on trade."
A charitable interpretation is that Trump doesn't think members of the Republican establishment need to align behind him in order for him to be successful. That was certainly true in the primaries, but it's less clear that it's true in the general. Why? For the same reason that the uncharitable interpretation of Trump's comments is so baffling: Trump very much needs Republicans to vote for him in November.
That sounds obvious, of course, but it's worth delving into. Consider, for example the relative unfavorability of Trump and Hillary Clinton within their own parties. Clinton's got the Democratic nomination essentially locked up, but is still battling Bernie Sanders and still maintaining only a small lead over him in national polling. But she is much more positively viewed by members of her own party than is Trump -- and consistently so. Trump's numbers have improved, but they're still pretty abysmal.
This is a large part of the reason that Trump's overall favorability ratings are lower than Clinton's: Republicans look at him a lot more skeptically than Democrats do Clinton. For him to be successful in November, he needs those skeptical Republicans to come out and vote for him anyway.
After all, this happens at a time when partisans have been more willing than ever to vote for the candidate their own party nominated. Even independents -- a group that largely still tends to vote on a partisan basis -- were largely loyal to the party with which they identified in 2008.
If Republicans waver on their choice but Democrats stay true to their party, Trump's in a lot of trouble. (Yes, a chunk of Bernie Sanders supporters say that they won't back Clinton in November, but when Clinton lost the nomination in 2008, the number of defections was much smaller than polling at the end of the primary suggested.)
Donald Trump will end the primary season with more votes from Republicans than any Republican in history. But he's also had the most people vote against him, as the splintered party struggled to reach consensus. The fact that prominent Republicans are reluctant to back Trump is a both a cause and side effect of that split. House Speaker Paul Ryan declining to endorse Trump won't hurt Trump among Trump's existing base of support; they don't like Ryan anyway. But if Ryan argued for Trump's candidacy -- if more moderate/establishment Republicans were to embrace and make the case for his nomination -- it's likely that wavering Republicans might be influenced. Trump needs them to be.
He waves this away by suggesting he'll find some space in the political middle. He returns to this baffling idea that he can lure Bernie Sanders's supporters to his cause -- an effort that will almost certainly fail based on the politics at play and an effort about which Sanders himself has been increasingly vocal.
There has been a repeated suggestion that Trump can lure Democrats to his cause in the way that Ronald Reagan did in 1980. (You can see the dip Reagan caused in Democratic Party loyalty on the first graph above.) But that idea is flawed for several reasons.
First of all, those Reagan Democrats -- mostly working-class white males -- have already migrated to the Republican Party. You can see the trend in data from the General Social Survey; the Reagan Democrats of 1980 are the regular-old Republicans of today. In that sense, Trump is right: His campaign hinges on those voters supporting him.
On top of that, though, white voters are a much smaller part of the electorate than they were in 1980. That year, 88 percent of the electorate was white. In 2012, the figure was 72 percent. In 1988, working-class whites made up half of the electorate, as the Atlantic's Peter Beinart noted in March. This year, they'll be only one-third. Yes, it's possible that Trump will inspire more whites to come to the polls, but there's also some evidence that he's inspiring nonwhites to turn out, too -- to vote against him.
Trump can't count on wooing a large group of Democrats to vote for him in part because most of the Democratic Party is made up of groups that view him very negatively: women, blacks, Hispanics. If he can't convince Democrats, and if Democrats and Democratic-leaning independents maintain their party loyalty, he needs every Republican vote he can get. To do that, he needs help -- the sort of help he didn't get in the primaries, leading to his earning less than 50 percent of the total votes.
It sounds macho to say he doesn't need loyalty, that he'll go it alone, with the party or without it. But a non-unified Republican Party is a Republican Party that endures four more years of a Democratic White House.