You can sum up Florida Sen. Marco Rubio’s biggest impediment to being chosen vice president in two words: David Rivera.

In this Nov. 2, 2010, file photo, Republican David Rivera speaks to supporters in Coral Gables, Fla. State prosecutors say they won't charge Rivera with a crime but the 16-page memo they issued Wednesday, April 18, 2012, outlines allegations of questionable financial practices which may make his re-election bid more difficult. (AP Photo/Alan Diaz, File)

“I think until David is federally cleared on any wrongdoing, it is damn near a [disqualifier]...we continually hear that though the state authorities took a pass, they did so knowing others would not,” said one senior Republican operative in Florida. “Marco is his friend, and he is loyal, but it comes to a point when stakes are so high and margins so close that it would be the difference between being invited to the ticket or not.”

On Monday, Rubio was asked about Rivera by Fox News Channel’s Bret Baier and here’s what he said:

“He’s a friend. I mean, he’s a friend I’ve known on a personal level even before I was elected or he was elected to office. So look, I know he’s going through a tough time. And we’ve all read the press reports and none of us like to see that about anybody, much less a friend. And he’s going to have to deal with those issues.”

Baier — a Fix friend — then pushed Rubio on Rivera, noting that not only is the Florida Senator hosting a D.C. fundraiser for his friend on May 16 but that the two men also co-owned a house together, a house that went into foreclosure.

Rubio sought to cast the foreclosure issue as a simple misunderstanding; “There was a disagreement with the bank about how much the monthly payments were,” he told Baier, adding: “And it all got confusing”.

Rubio’s tack toward Rivera to date is to acknowledge that, yes, the House Member is troubled but that the two of them have been friends for a very long time and it wouldn’t be right to abandon Rivera now.

In essence, Rubio is appealing to people on very human grounds — trying to tap into the fact that most of us have a friend (or several friends) whose conduct we don’t totally condone. (Or, to paraphrase Bruce Springsteen, “when it’s your brother, sometimes you look the other way”.)

The problem for Rubio is that while his point is broadly true — the Fix has a few friends like that — not everyone is a national political figure who is at or near the top of most lists of who former Massachusetts governor Mitt Romney will pick as his vice presidential running mate.

It’s not clear how long Rubio can keep this “he’s an old friend” schtick up — and there are clearly political perils in doing so. Rubio will wind up answering for Rivera at every turn and, in the event the IRS and FBI decide to prosecute, he’ll have to get as far away from his friend as quickly as possible.

(Sidebar: If President Obama could do it all over, don’t you think he would have severed ties completely from Rev. Jeremiah Wright as soon as his one-time preacher became an issue?)

What the Rivera relationship reveals is that Rubio remains a somewhat lightly vetted candidate on the national stage. Why? Because the story of the spring and summer of 2010 was the implosion of Gov. Charlie Crist’s Senate candidacy — and political career. Crist’s collapse in the Republican primary and then much-publicized decision to run as an independent blotted out the political sun in the state.

To the extent there was a Rubio storyline, it was that he was the conservative kingslayer — Jaime Lannister reference! — and a rising national star. Need evidence? The story of Rubio’s misuse of a state party controlled credit card — on Monday he acknowledged “I shouldn’t have done it that way” — barely made a ripple in the race.

The narrative was set. Crist was the loser. Rubio was the rising star. There was no room in that story for questions about what Rubio had done during his time in the state House.

The fact that outside of the Florida papers there has been relatively little written about the Rubio-Rivera relationship is indicative of that inch-deep vetting. That won’t continue as Rubio continues to be touted as either a) Romney’s vice presidential pick or b) the odds-on favorite to be the Republican presidential nominee in 2016 or 2020. (You’ve already seen evidence of the deeper dive into Rubio with questions raised about his remembrance of his parents’ decision to flee Cuba.)

It’s worth noting that some independent observers believe that the Rivera-Rubio connection alone is not enough — at least at this point — to be a major problem to the Florida Senator’s vice presidential prospects.

“To date, I don’t think there is enough there to disqualify him,” said Brad Coker, who heads up the Jacksonville-based independent polling firm Mason-Dixon Polling & Research. “Outside of South Florida and state political insiders, few here are even aware of it.”

Still, if the foundational principle of picking a vice president is “do no harm” than it’s not difficult to imagine the Romney folks being a bit spooked by the Rubio-Rivera relationship and what else they might not know about the junior Senator from Florida.

And in the veepstakes, like in a court of law, you don’t have to prove guilt or innocence, you just have to prove reasonable doubt. If picking Rubio starts to look/feel like a risk, expect the Romney people to steer clear.