Marco Rubio has been riding weeks of great buzz in the GOP race — but with the Iowa caucuses getting closer and closer, the Republican Party still hasn't fallen behind him. A new book suggests one possible reason why: Allies of Jeb Bush have quietly smeared him behind the scenes.

The Republican establishment's failure to unite around Rubio — who seems to most observers to be the strongest general election candidate in the GOP race — has seemed puzzling to many, particularly when more elites are worrying that Donald Trump could actually win. Though Rubio's picked up a few congressional and big-donor endorsements since Bush's campaign went off the rails, he remains behind Bush in FiveThirtyEight's endorsement tracker. And Bush himself had won a historically low amount of endorsements for a putative frontrunner.

So the story of the Republican "invisible primary" — the contest for the support of party elites and actors — is still, as I wrote in September, mainly one of party indecision. And there are many possible reasons for this. Perhaps the party is worried that Rubio wouldn't be able to beat Trump one on one, or that a unified effort to back him could be counterproductive. Perhaps Rubio simply hasn't built up the personal ties that would lead elites to go out on a limb for him. Or perhaps the party's ability to "decide" isn't as sweeping as some have suspected.

But it's worth remembering that much of the invisible primary is, well, invisible. Conversations happen behind closed doors that we're not privy to. And one difficulty Rubio has faced all year — for several years, really — is that politicos in his home state have spread lots of salacious — and completely unsubstantiated — rumors about him.

A new book claims that Jeb Bush's allies have been spreading rumors about Rubio

Since there's no evidence that these rumors about Rubio have any truth to them, the mainstream press has tended not to mention them in print. But a whisper campaign like this could well lead GOP elites to be hesitant to fall behind Rubio for fear of a looming scandal.

McKay Coppins of BuzzFeed News provides a glimpse into this process today, in an excerpt from his new book The Wilderness. According to Coppins's account, Rubio's team has fought to bat down two rumors that are particularly persistent: one involving an affair, and another that Rubio is "hiding a secret second family." Coppins reports that allies of Jeb Bush tried to prevent Rubio from running in the first place by "circulating the rumors anew among donors and politicos," and that they "tried to convince a number of influential figures in political media that they had the goods on Rubio."

To be clear: There is absolutely nothing to substantiate any of these rumors. Rumors flew about Barack Obama when he was first running for president in 2008. There was, of course, the infamous allegation that he wasn't truly born in the US. Then there was the claim that he was a secret Muslim. Then there was the "whitey tape" — a supposed tape somewhere out there in which Michelle Obama derogatorily referred to "whitey," which was certain to tank Obama's campaign as soon as it appeared. (Seven years into Obama's presidency, we're still waiting for the tape to materialize.)

Indeed, there are also many reasons to doubt these particular rumors (besides, of course, the fact that they're simply anonymous gossip). Rubio won a hard-fought Senate race in 2010 against the governor of Florida, and is now facing some very well-funded opponents in his presidential bid. Allies of Charlie Crist, Jeb Bush, and Ted Cruz were all surely eager to dig up whatever dirt they could find on Rubio, and likely spent lots of money trying to do just that. Yet so far they've come up with nothing that we know of. Coppins also writes that Rubio's top aide, Terry Sullivan, paid a political research firm to investigate the rumors, and the firm "determined that they lacked concrete evidence."

Still, as with the unsubstantiated affair rumors that helped derail Kevin McCarthy's bid for speaker of the House, the mere existence of this gossip plays a role in the GOP race behind the scenes. Look at this slide from a Bush campaign presentation to donors, which was obtained by David Catanese of US News in October — and note the last bullet point in particular:

Again, no evidence presented for that last bit, just shady allusion that there's something out there! Since Bush's own campaign is looking so beleaguered, he and his allies could well see rumor-mongering about Rubio as his last chance. (Bush spokesperson Tim Miller denied to Coppins that the campaign had spread the rumors.)

But overall, when watching the race it's worth remembering that the conversations GOP power players are having as they weigh whom to fall behind aren't public. Though it's highly unlikely that they know anything about Rubio that we don't — again, if there were any actual evidence backing up these rumors, we'd probably know about it by now, or will very soon — whispers and gossip could well weigh on their decision-making process.

VIDEO: Marco Rubio shuts down Jeb Bush during a debate