Hurling insults at a fellow Republican is one thing; refusing to support the party nominee is another. Photo: J Pat Carter/Getty Images; Branden Camp/Getty Images

On September 3, 2015, many Republicans heaved sighs of relief when Donald Trump signed a pledge saying he’d support the GOP nominee for president instead of entertaining a third-party candidacy. In order to secure that pledge, all of the other candidates signed it, too, which wasn’t surprising since everyone but Trump had promised to support the nominee during the very first candidate debate.

Trump was pretty clear that he’d honor the pledge so long as the GOP treated him “fairly,” which most obviously meant he’d have firm party support if he did win the nomination (still not something many Republicans seriously thought possible last September).

And at nearly every subsequent Republican debate some candidate would ritualistically opine that every one of them would be a better president than the godless socialists running for the Democratic nomination — and nobody publicly dissented.

The tone of the GOP competition obviously got very toxic during the most recent candidate debate in Houston, and the surprise aggressor at that event, Marco Rubio, hasn’t let up in the days leading up to today’s Super Tuesday megaprimary. But while everyone’s been focused on how the candidates might fare today on the vast battlegrounds of primaries and caucuses, Rubio crossed a line: embracing a #NeverTrump movement that’s an implicit, if not explicit, violation of the loyalty pledge. If there’s any doubt about Rubio’s new attitude toward his bitter rival, his rhetoric has removed it. After Trump temporarily failed to denounce (or, depending on how you read the chain of events, re-denounce) former Klan leader David Duke, Rubio hurled this anathema:

Marco Rubio, who has been savaging Trump as a “con man” for three days, responded by saying that Trump’s defiance made him “unelectable.” The senator from Florida said at a rally in Northern Virginia, “We cannot be the party that nominates someone who refuses to condemn white supremacists … ” Rubio and his aides have been promoting a #NeverTrump campaign on Twitter.

Trump has certainly chosen to interpret never as meaning “never,” and he’s responded with his own threat:

In response to all this, Trump told Jake Tapper that his loyalty pledge was “a two-way street,” and “they’re gonna have a problem, they’re gonna have a big problem with me.”

Independent or third-party candidacies aside, it’s very, very rare for major presidential candidates who lose nomination contests to refuse to endorse the winner. The only two I can think of are Nelson Rockefeller, who never got around to endorsing Barry Goldwater in 1964, and George Wallace, whose recovery from an assassination attempt protected him from recriminations for failing to provide what would have been a very unlikely endorsement to George McGovern in 1972.

Hmmm. Barry Goldwater and George McGovern … what is it they have in common?

Yes, this could be a pretty significant moment in this most agonizing of presidential cycles for the GOP. Perhaps cynics expect Trump and Rubio to kiss and make up when the dust has settled and raise hands together in Cleveland. But it’s reasonably clear that Rubio has decided not to let Republicans off the hook by professing neutrality in the presidential race, and that he’s more than willing to counter Trump’s threats to a united party in November with his own.