One of the nagging questions of the Republican primary has been why the GOP establishment hasn't united behind Marco Rubio. The move seemed obvious — they feared Donald Trump, they loathed Ted Cruz, and Rubio seemed like a more serious threat to Hillary Clinton than Jeb Bush or Chris Christie.

But it didn't happen. And it kept not happening. Even as Bush tanked and Christie struggled, Rubio got a few endorsements, but never the flood that would have signaled GOP elites were closing ranks; he raised some money, but nothing that approached Bush's early haul.

The reason for the Republican Party's reticence to back its best prospect became sort of a guessing game in punditry. Some attributed it to rumors that Rubio had skeletons in his closet — perhaps the Romney team had uncovered some serious wrongdoing when they vetted him, or maybe it was the persistent (and completely unsubstantiated) whispers that Rubio had a second family hidden away somewhere (whispers that the Bush team apparently encouraged). Others suggested it was Rubio's tendency to knife his colleagues in the back, or to disown bills he had co-sponsored.

When I talked with operatives of rival campaigns, they said the case they were making was simpler. Rubio, they said, wasn't ready. He was a stump speech attached to a pretty face. He had no accomplishments and no executive experience, and he was going to fall apart under pressure. They argued that pundits who dipped in and out of the campaign thought Rubio was a good speaker, but if you watched him closely you learned he only had that one speech, and he delivered it the exact same way every single time.

I didn't take this critique very seriously. Presidential campaigns don't actually reward experience — if they did, John McCain would have wiped the floor with Barack Obama, and if he didn't, it would only have been because Hillary Clinton did it first. As for the criticism that Rubio was too reliant on his excellent stump speech, that seemed like sour grapes from campaigns that were desperately searching for even one speech that connected with voters.

And then Saturday's debate happened. What Christie did at the debate was take the case being made quietly against Rubio and shout it into a microphone. He said, aloud, that Rubio had no accomplishments, no executive experience, and nothing to fall back on except the robotic repetition of his stump speech. And then Rubio, disastrously, fell back on the robotic repetition of his stump speech. And then he did it again. And again. It was like his "repeat" button jammed.

There's a good case to be made that Rubio's glitch at the debate won't matter. As my colleague Andrew Prokop notes, Cruz was thought to have had a bad debate right before the Iowa caucuses, but he won anyway. And who knows? Maybe Republican voters agree with Rubio that it's of paramount importance to establish that Obama is an evil genius rather than a bumbling fool.

But I think it will matter, and the reason it will matter is that this is what the other campaigns have been privately saying about Marco Rubio all along — that he just isn't ready to be the nominee. Before Saturday, it was a convincing enough message that the Republican Party hadn't united around Rubio, despite the obvious benefits of doing so. After Saturday, the argument has a lot more force.

Gaffes matter when they confirm underlying doubts about a candidate. That's why Rick Perry's "oops" moment echoed so far and wide — it validated suspicions that Perry wasn't quite up to the rigors of the campaign. If the same thing had happened to Romney, it would've been a one-day story, because Romney was a PowerPoint presentation reincarnated as a human being — no one believed he couldn't remember a bullet-pointed list of three items.

Rubio's stumble on Saturday was an "oops" moment; it confirmed underlying doubts about his candidacy — doubts that the rival campaigns have been whispering in Republican ears for months now, with surprising success. If you're a Republican donor today, you're not looking to push Christie and Bush and Kasich out of the race so you can ensure Rubio gets to stand on a stage debating Hillary Clinton. Instead, you're less sure than ever whether you want Rubio on stage with Clinton at all.

5) The big question about Rubio is: can this untested novice cope with the demands of the presidency? — David Frum (@davidfrum) February 7, 2016

The hope for Team Rubio was that they would build on their surprisingly strong third place in Iowa with a surprisingly strong second place in New Hampshire — and the dual displays of competence and momentum would help persuade the Republican Party to do the obvious thing and unite behind Rubio's candidacy. But now Rubio has raised doubts inside the Republican Party and hopes inside the other campaigns, and the party is more worried about uniting behind him than ever.

This doesn't mean Rubio is finished, of course. He's hardly the first promising primary candidate to stumble amid the heat of the race, and this is hardly the worst crisis a promising candidate has ever faced. In 2008, Obama was caught on tape saying that rural voters get "bitter" and then "cling to guns or religion or antipathy to people who aren't like them." In 1992, Bill Clinton faced the revelations of an extramarital affair with Gennifer Flowers on the eve of the New Hampshire primary.

If Rubio really is as good a candidate as he's seemed at certain times in this race, he has plenty of time to prove that Saturday night was an aberration and win the nomination. But insofar as he was hoping to unite the party around him after New Hampshire, that's no longer going to happen.

Correction: