At last night's debate in Detroit, each candidate reiterated his promise to back the party's nominee even if it's Donald Trump—or, in Trump's case, to back him even if it's someone else. That won't necessarily prevent a major third-party or independent challenge this November: Anti-Trump Republicans could still go to war with a candidate not named Cruz, Kasich, or Rubio (if they have any sense, they'll prefer it that way), and Trump probably figures his campaign can file for bankruptcy and renegotiate its contractual commitments. But together with the big ballot-access barriers that a splinter candidate would face, last night's comments are a reminder that a three-way race is far from inevitable. We won't necessarily see a rerun of the Republican split of 1912 or the Democratic fracture of 1948.
That said, a split can manifest itself in other ways. Consider the last two times a major party picked a candidate that much of the party's leadership rejected.
In 1964, the Republican Party was coming off a long stretch of nominating relatively moderate presidential candidates—Dwight Eisenhower, Thomas Dewey, Wendell Willkie—and beating back Old Right challenges from the likes of Robert Taft and John Bricker. Then Barry Goldwater won the nomination with a program that combined the Old Right's skepticism about the New Deal order with a super-hawkish foreign policy; he also angered liberal Republicans by opposing the Civil Rights Act of 1964.
In virtually any presidential election, some people will defect from their party. But this time the defectors were a lot more prominent than usual, with men like New York Gov. Nelson Rockefeller and Michigan Gov. George Romney—Mitt's father—refusing to endorse or campaign for Goldwater. Sensing an opportunity, the Democrats started running ads like this one:
There weren't any major third-party campaigns that year, just a bunch of Republicans who decided to either throw in with the Democrats or stay home. Goldwater received a mere 38.5 percent of the popular vote.
America avoided the nuclear war invoked in that commercial, but it did get a vast escalation of the war in Vietnam. In 1972, one of Washington's fiercest opponents of the Vietnam conflict—South Dakota Sen. George McGovern—captured the Democratic Party's nomination. Once again, powerful forces in the party withdrew their support, most notably when the AFL-CIO declined to endorse him.
As in 1964, the admen on the other side rolled out a welcome mat for party defectors. A group called Democrats for Nixon—allegedly independent, in fact funded by the Committee to Re-Elect the President—produced ads like this one, which upbraided McGovern for the allegedly crippling cuts he wanted to make to military spending:
Again, there was no major third-party challenge, or at least not among disaffected Democrats. (On the right, a campaign by the Bircher congressman John Schmitz attracted over a million votes.) Again, there didn't need to be. When an institution like the AFL-CIO—ordinarily a key part of the Democratic coalition—refuses to endorse a candidate, that has an effect. Even some of the Democrats who did back McGovern clearly didn't have their heart in it. Chatting on the phone as the returns came in, Nixon told the previous Democratic nominee, Hubert Humphrey, that he said he understood why he'd campaigned for the president's opponent. "Well," Humphrey replied, "I'll have to have a talk with you sometime….I did what I had to do. If not, Mr. President, this whole defeat would have been blamed on me and some of my associates."
When the ballots were tallied, McGovern did even worse than Goldwater did, getting only 37.5 percent of the vote.
If Trump is nominated, will he face those sorts of high-profile defections? For a lot of Republican officials, it'll depend on whether they think he has a good chance to win—they won't want to alienate a potential president, but they won't want to go down with a sinking ship either. Even if someone doesn't distance himself formally, he can still back off in practice. Ted Cruz and Marco Rubio may stand by their pledge to support their party's nominee, but that doesn't mean they'll actually campaign for the guy. (Or that they won't campaign half-heartedly, a la Humphrey.) In the pundit class, some prominent neoconservatives have already declared they'll back Clinton over Trump if it comes to that, mostly (though not entirely) because they prefer her foreign-policy views. It remains to be seen whether that will move any more voters than Colin Powell's two endorsements of Barack Obama did.
There's one thing we can be sure of. No matter how well the Republican elites manage to hang together, if Trump is the nominee the Clintonites will start shooting a sequel to the Republicans-for-Johnson and Democrats-for-Nixon ads.