So, that happened. Earlier this week, The Washington Post reported that Marco Rubio's campaign had been "promoting a #NeverTrump campaign on Twitter." That hashtag, which has occasionally trended since the Feb. 25 debate, is the cri du coeur of conservatives who have vowed that they will not support Donald Trump under any circumstances. For Rubio, signing on was the abrogation of a pledge he'd previously made to support the eventual nominee, and it gave rise to the notion that he -- or another nominee -- might be willing to take the fight with Trump to the Republican National Convention in Cleveland.

But at Thursday night's debate, Rubio backed down.

The moment came toward the end of the debate. Here's how it went down:

BRET BAIER: Gentlemen, this is the last question of the night. It has been a long time since our first debate, seven months ago in Cleveland. A lot has transpired since then, obviously, including an RNC pledge that all of you signed agreeing to support the party's nominee and not to launch an independent run. Tonight, in 30 seconds, can you definitively say you will support the Republican nominee, even if that nominee is Donald J. Trump? Senator Rubio, yes or no?

RUBIO: I'll support the Republican nominee.

BAIER: Mr. Trump? Yes or no?

RUBIO: I'll support Donald if he's the Republican nominee, and let me tell you why. Because the Democrats have two people left in the race. One of them is a socialist. America doesn't want to be a socialist country. If you want to be a socialist country, then move to a socialist country.

The other one is under FBI investigation. And not only is she under FBI investigation, she lied to the families of the victims of Benghazi, and anyone who lies to the families of victims who lost their lives in the service of our country can never be the commander- in-chief of the United States.

For Rubio, who's led the belated charge against Trump -- deploying broadsides that have ranged from pointed attacks on his policies (or lack thereof), his business career, and his face -- this was an abrupt departure from the line he'd seemed to have drawn in the sand the moment he started vowing to not let the conservative movement fall into the hands of a "con artist."

Ted Cruz and John Kasich offered a similar answer, saying that they, too, would support Trump as the nominee. But it was the Florida senator who had seemed to have gone all-in on opposing Trump by any means necessary -- until he suddenly wasn't.

It's important to remember that going into Thursday's debate, the passion to keep Trump from winning the nomination had led certain factions of the GOP to start contemplating some radical solutions .

Rubio was at the center of these dreadful imaginings. As Daily Intelligencer's Ed Kilgore remarked , Rubio had become the candidate threatening the "hostile takeover" of his own party, by "hinting at some sort of monstrous convention cabal to stop Trump if voters refuse to do so." (By contrast, Cruz had continued to aver that he'd support the nominee, whoever it is -- but that he'd prefer Rubio and Kasich quit the race so it'd be him.) Things have only escalated since then: As CNN reported , former presidential nominee Mitt Romney is supposedly exploring how he can get into the contested convention game.

So, in the hours that led up to this debate, the antipathy of the anti-Trump forces had forced everyone to contend with the idea that someone might trigger the nuclear option of a nomination heist on the convention floor to stop Trump. During the debate, Rubio pursued his criticism of Trump with the same zeal as he did seven days ago -- adding pointed observations of Trump's failure to answer any question forthrightly to the mix. It all seemed to be building to the moment at which Rubio would declare that his opposition to Trump was total, offering himself as the avatar of the #NeverTrump movement.

Instead, he backed down. A cynic might wonder if at some point down the line, many of Trump's most fervent opponents, for all their talk of being willing to do the unthinkable at their Cleveland confab in July, aren't likely to do the same.