Picking the right one to run with Romney

July 5, 2012, 2:43 PM UTC

FORTUNE — Several months ago, we met a CEO who had one main complaint about his job. It wasn’t foreign competition or fickle consumers. No, it was loneliness. “I make every decision by myself,” he moaned.

“That’s nuts!” was our immediate reply. “You can’t run the place that way!”

Every leader needs a team, and every leader benefits enormously from having a wingman, a partner who can be counted on to counsel, goad, provoke, listen, and on and on. It’s true in business, and it’s true in politics. A great person in the No. 2 spot can make the person at No. 1 decidedly stronger, smarter and more effective.

So, Mitt Romney, don’t blow the VP thing.

Fortunately, that would be pretty hard at this point. The current short list of contenders easily passes muster in terms of intellectual heft and leadership experience: Florida Senator Marco Rubio, Senator Rob Portman of Ohio, Representative Paul Ryan of Wisconsin, Governor Chris Christie of New Jersey, and Governor Bob McDonnell of Virginia. There’s not an iffy option in the bunch.

MORE: Dear America: It’s time to up our entrepreneurial game

So how should Romney choose among them? To answer that question, we’ve put together a scorecard that rates each candidate against six criteria we consider essential in a vice-president.

Of course, it’s possible none of these individuals will be Governor Romney’s final pick. Both Dan Quayle and Sarah Palin took most everyone by surprise, to put it mildly. But even if Romney were to select a wild card like Condoleezza Rice – who has loudly disavowed any vice-presidential aspirations – the selection criteria should remain the same:

First, a VP must be able to be president, not eventually, but from Day One. The main, unfortunate reason for this doesn’t need to be said. But such a requirement is also about creating confidence in government. The right VP pick sends a message to America and the world: No matter what happens, the bottom will never fall out of our system.

Again, all of the current VP contenders could serve in the Oval Office without hesitation, but Portman’s nearly 20 years of government service and Christie’s and McDonnell’s tenures as chief executives of their states give them an edge on this count.

Second, a VP can’t be a clone of the president. There’s just no point. A VP’s value comes from bringing different skills and abilities to the table – as meaningful as they are complementary. Rubio, for example, would contribute inspiring oratory to Romney’s repertoire; Ryan an intricate understanding of the more-important-than-ever budget options; and Portman a thorough understanding of Washington’s inner workings. Christie would add a level of excitement and humanity to the White House. Bottom line: With the exception of McDonnell, who’s very similar to Romney, the candidates are all about equal on this criterion.

MORE: Howard Schultz: “The American dream is in jeopardy”

Third, a VP has to have guts. Why? Because it’s every No. 2 person’s profound responsibility to look their boss squarely in the eye and deliver the hard, awkward, unpleasant and even painful messages that no one else can. “This is what people are thinking but not telling you.” Or, “You might have come off too strongly in that meeting. To some people, it probably looked like you weren’t listening.” The VP, in other words, needs a certain fearlessness, an attitude born of self-confidence and candor. And here, while everyone under consideration certainly seems up to the challenge, the advantage has to go to Christie, who, to our knowledge, has never managed to mince a word in his life.

Fourth, a VP has to project gravitas and be a significant presence, but cannot overshadow the president. This criterion is (again) about confidence in the system. The leader needs to be the focal point for the nation – fully aware and in charge. Optics matter. And on this count, only Christie, with his unfettered personality, poses a real problem for Romney.

Fifth, the VP has to be a real partner to the president – keeping confidences and blocking any attempts from below to divide and conquer. Remember, the White House is an organization like any other, filled with politicking and intrigue. And, as in any organization, you cannot have a presidency where the VP’s office is a place to shop ideas or slip initiatives through. Even if they disagree in private – which they should – in public, the VP and president must stand as one. Again, Rubio, Portman, Ryan and McDonnell seem comparable on this front; Christie is just too used to being No. 1.

MORE: Sizing up automotive CEO pay

Finally, a VP should ideally help bring a critical constituency into the fold. This is an election, after all! That said, no one really knows if Rubio can capture the Hispanic vote in significant numbers, or if Portman can guarantee a return of Ohio to the GOP, or if Ryan and McDonnell can help in what will be very close races in their states. We’re not pollsters, but giving it our best guess: advantage Rubio.

Put it all together, and as you can see from the graphic, our criteria suggest that Rubio and Portman are the best choices for vice-president, but only by a fraction. If he picks from the current crop, we’d say Romney really can’t go wrong.