Donald Trump had a good night on March 15, but polls show Republican party isn't entirely behind the GOP front-runner. (Peter Stevenson/The Washington Post)

Donald Trump's insistence that he would win in November against the Democratic candidate is usually offered with the explanation that he'd win states that Republicans keep losing. He'd win New York, Ohio, Pennsylvania, Florida, you name it.

There's just one problem: Preliminary exit polls reported by ABC News and NBC News on Tuesday night suggest that even Republicans in Florida and Ohio aren't sure they'd want to back Trump in a general election -- at least, not if there were a third party.

That's a third of the Republican electorate that would be open to voting for a third party than Trump in a general election, based on early numbers. According to ABC, two-thirds of those who now oppose Trump fall into that camp. Part of that is the result of the contested primary season. Consider that 2 in 10 Florida Republicans wouldn't back Marco Rubio if he were his party's nominee, and about the same number say that of Ted Cruz across the states voting on Tuesday. People like their candidates and don't like the candidates that are trying to beat them.

Trump, of course, is a unique beast. He's weathered a coordinated attempt to undermine his candidacy; more than $35 million has been spent trying to beat him to date. The bulk of that was spent in Florida which, barring something remarkable happening, Trump will likely win handily.

On Tuesday morning, Politico reported that a group of prominent conservatives was trying to develop a strategy to run a third-party candidate against Trump, should he win the nomination. The plan is in the "embryonic" stage, according to Politico's Shane Goldmacher, but perhaps there is at least an option to give those Republicans looking for a third party something to look at.

Or, perhaps not. When Michael Bloomberg was still considering a third-party bid, MSNBC's Benjy Sarlin looked at what it would take to get on the ballot. In Texas, for example, an independent candidate would need tens of thousands of signatures by May 9 in order to get on the ballot. A more likely scenario would be finding a candidate who could run on a party line that's already likely to qualify for the ballot in enough states to make a difference. Sarlin points out that the Libertarian Party will likely hit that mark -- meaning that conservatives could find themselves in the unusual position of encouraging Republicans to avoid the Republican candidate for president and vote for the Libertarian, at least this once. If they find a candidate. And if everything else falls into line.

The real question here isn't if Trump would be battered by a strong third party candidate. It's how many Republicans in Florida and Ohio -- which he pledged to win -- would skip voting altogether rather than vote for Trump. In preliminary exit polling Tuesday night reported by ABC, 4 in 10 non-Trump voters say they wouldn't back him in November.

But is that reliable? In 2008, fans of Hillary Clinton pledged to not vote for Barack Obama in the general; in the general, Democrats backed Obama strongly. A better analogy may be the addition of Sarah Palin to the Republican ticket in 2008. As we noted earlier this year, a study found that Palin's vice presidential nomination cost the party 2 million votes in the general. This is an estimate, of course -- but it suggests that voters will in fact stay home if they don't like the candidate.

Trump has to get the nomination first, of course, which is a challenge of its own. If he doesn't, maybe he could be the Libertarian candidate. Weirder things have happened in the last 12 months.