Mitt Romney won’t make his vice presidential pick for a few months yet. But if history is any guide, we already know who he will choose: a conservative.

Why, you ask? After all, although Romney’s struggles with the most conservative wing of the party have been on full display during the almost-but-not-quite-yet-over presidential primary race, he’s managed to emerge as the likely nominee despite that weakness. And although Romney has lost every state in the deep South during his march to the nomination, no one in either party expects him to have to fight President Obama for victories in Alabama, Mississippi and the like.

Given all that, it makes a certain kind of sense that Romney’s vice presidential pick will be determined primarily by geography, camaraderie and readiness for the top job, not ideology. And it might. But making that sort of non-ideological choice would represent a major break with how recent Republican nominees have made up their minds about their second-in-command.

Let’s start with the most recent veepstakes pick and work our way backward. Arizona Sen. John McCain (R) had sewn up the nomination by this time in 2008, but he still had a major problem among conservatives, who distrusted his past apostasies on issues such as campaign finance reform. That made McCain’s preferred pick — Connecticut Sen. Joe Lieberman, an independent — a non-starter. McCain eventually decided to shore up his ideological right flank by picking a conservative governor named Sarah Palin. (The rest of that story is, as they say, history.)

Eight years before that, Texas Gov. George W. Bush had championed “compassionate conservatism” on the way to the Republican nomination. Although former Wyoming congressman Dick Cheney wasn’t a direct appeal to the ideological right, Cheney was a very consistent conservative and a known presence in those circles. And it’s hard to imagine Bush having picked someone to his left ideologically.

In 1996, a similar pattern played itself out. Kansas Sen. Bob Dole, a pragmatic dealmaker at heart, chose former New York congressman Jack Kemp, a fiscal hawk’s fiscal hawk, as his vice presidential nominee. In 1988, Vice President George H.W. Bush went ideological with his pick, too — plucking Sen. Dan Quayle of Indiana, a conservative favorite, from relative obscurity to run with him on the ticket.

The ideological matching can work the other way, too. Ronald Reagan, a conservative firebrand, went with Bush, the epitome of the establishment, when choosing his vice president in 1980.

Why do Republicans feel the need to find ideological equilibrium when making their vice presidential pick?

“Historically, and presently, the central fault line inside the GOP has been between the moderate/establishment wing and the movement conservative wing,” explains GOP consultant Jon Lerner. “The choice of running mate has very frequently been used to unite the two wings.”

Put simply: To heal the one main rift within the party and truly excite the entire GOP about the fall election, the vice presidential pick has to be your ideological opposite (or close to it).

If you subscribe to the ideological argument when it comes to the veepstakes — and it’s hard not to, given the recent examples — then the number of choices for Romney is significantly narrowed from the dozen or more names being bandied about.

The key is that the pick would have to be considered a conservative by conservatives’ own standards — someone with the record and the temperament to placate any lingering doubts about Romney. It would have to be someone whom conservatives would be genuinely excited about.

Three people currently mentioned in the great process that is the vice presidential sweepstakes fit that description to a T: Sen. Marco Rubio of Florida, Louisiana Gov. Bobby Jindal and New Jersey Gov. Chris Christie.

Rubio is the darling of the tea party movement, riding into office in 2010 on a wave of national publicity that cast him as the next big conservative thing. Jindal was reelected with nearly 70 percent of the vote in November and would be a history-making choice as the first Indian American on a national ticket. Christie has become a national sensation since his election in 2009 as a speak-truth-to-power (and the media) conservative.

Pick one of those three men. If history is a guide, you have a 33 percent chance of telling all your friends that you picked the vice presidential nominee months before Romney announced it.