What's up between Marco Rubio and Joe Scarborough? Once friendly — on the air, at least — the Republican presidential candidate and MSNBC host are in what the New York Times on Monday termed an "unusual spat."

Scarborough, one of few conservative commentators on a cable channel favored by progressives, has often criticized and occasionally mocked Rubio, who refuses to appear on the "Morning Joe" program. The Times article included a suggestion by veteran Florida political reporter Brian Crowley that Scarborough might be jealous of Rubio's success. Scarborough is a former four-term congressman from Florida, and Rubio is the state's junior senator.

The TV host told the Times that he doesn't "know Marco well enough to resent him" — the two have never met in person, he wrote in Politico on Saturday — and he objected to the story's premise on Twitter Sunday night, after it appeared online ahead of Monday's print edition.

There's no doubt that Scarborough has been tough on Rubio. The senator's shortcomings in Scarborough's book range from inexperience on the national stage to bad taste in footwear.

And it was Scarborough's prodding of Rubio backer Rick Santorum last week that produced one of the most memorably unflattering moments of the campaign so far; Santorum seemed unable to name a single legislative accomplishment by Rubio.

When the conservative National Review magazine explored the obvious friction between the two men in November, it traced the origin to Rubio's Senate run six years ago, "when Scarborough argued that Rubio was too young, too unseasoned, and — perhaps most seriously — too compromised by ethical issues to win the Senate seat."

The first flare-up was on April 21, 2010. The night before, on April 20, the Miami Herald reported on a criminal investigation into the use of credit cards issued by the Florida GOP. The report cited an anonymous source alleging that the Internal Revenue Service was combing through Rubio’s tax records, along with those of two other Florida Republicans — which the Rubio campaign furiously denied, and which has never been confirmed. The following morning, Scarborough led his show with the story, alleging that it might be merely the first of several forthcoming disclosures that could threaten Rubio’s campaign.

Yet Rubio, who had appeared at least three times on "Morning Joe" before that Miami Herald story, rejoined Scarborough soon after and showed no sign of displeasure with his treatment. In fact, he praised Scarborough and co-host Mika Brzezinski for broadcasting from Pensacola Beach in the aftermath of the Gulf oil spill.

"Thank you for doing this program from Pensacola," Rubio said. "The images you're showing this morning and the attention you're giving to this is an extraordinary, I think, boost to the spirits of the people in Northwest Florida."

Roughly a year after his election, Rubio was back on "Morning Joe" (though Chris Matthews was filling in for Scarborough that day). It wasn't a contentious interview.

So, if Scarborough's criticism of Rubio isn't new, why is the senator complaining now? I suspect the reason is that Scarborough personifies the GOP "establishment" argument that Rubio — 44 and still in his first term — is too green to be president. The National Review reported that at an October gathering of top Republican donors in New York, Scarborough said it is "not his turn" and asked "why doesn't he stay in the Senate and learn something and run in 2032?"

At a moment when Rubio is being branded by the press as the establishment favorite, following his strong showing in last week's Iowa caucuses, griping about Scarborough seems like an attempt to show Republican voters that he's not so beloved by the establishment after all. This is the year of the outsider, after all, and Rubio appears to be grasping for a bit of rebellious street cred.

In an interview on Fox News Channel last week, Rubio made clear that he doesn't want to carry the establishment mantle.

SEAN HANNITY: I don't think, in this insurgency year, you want to be labeled "establishment." Am I right about that?

RUBIO: Yeah, well, it's not an accurate label. I reject all these labels. Those are things the media comes up with because it makes it easier for them to cover the political news.

I'm sure that Rubio hasn't appreciated some of the things Scarborough has said about him. But he's managed to ignore them until now. Taking issue with Scarborough now doesn't seem personal. It looks like political strategy.