For the presidential candidates, it is too early to talk about vice presidents, but not too early to think about them.

It’s still premature to talk because truly, the races are yet to be run. The Florida primary isn’t decided for another week. Then comes Supercalifragilistic Tuesday, Feb. 5, when more than 20 states, including California, hold primaries. By then, the smoke should have cleared in the Obama-Clinton race and the unpredictable affair that is the Republican nomination.

So who should be number two? In the past, the key factor in choosing a vice-presidential running mate often has been geography. Nominees choose partners from across the country to balance the ticket. Other efforts at balance might be ideological, or based on political background.

For example, in this election, should former Arkansas Gov. Mike Huckabee or former New York Mayor Rudy Giuliani win the Republican nomination, they might need to reach across the GOP tent to find someone who would strengthen their appeal to the party’s center as much as a corner of the country.

As for political background, in 2000 then-candidate George W. Bush eschewed geographical considerations — but quieted some pre-election criticism about his foreign policy and Washington experience — by tapping Washington insider (and former Wyoming congressman) Dick Cheney.

The geography angle is a little muddy in 2008. Sen. Hillary Clinton represents the liberal northeastern state of New York, though she’s from Arkansas. If she’s looking for a southerner to join her, why, just two steps down in the polls is former North Carolina Sen. John Edwards, who was on the Democratic ticket last time around. But would he really gain her any votes?

Last week, when Hillary’s husband was spotted in Napa, one of my colleagues passed along the rumor that Clinton, if nominated, would choose a running mate from Ohio, the battleground state in 2004.

Like Clinton, Mitt Romney is also of mixed regional heritage. He was governor of Massachusetts, which almost certainly won’t go for him in November, though his father was governor of Michigan, which might. Either way, he may want to look south or west for a partner. Similarly, Sen. John McCain, R-Ariz., might have to go east, and Sen. Barack Obama may have to go beyond a liberal northern stronghold.

Geography doesn’t always hold — Bill Clinton, from Arkansas, won with Al Gore, of Tennessee — but it usually does. For example, Richard Nixon’s two running mates were Henry Cabot Lodge, Jr. of Massachusetts and Spiro Agnew of Maryland. When Nixon lost to John F. Kennedy, Kennedy’s running mate was Texan Lyndon Johnson. When Nixon topped Minnesotan Hubert Humphrey in 1968, Humphrey’s running mate was Edmund Muskie of Maine.

I recently ran across a memorable quote from Humphrey about the lot of the vice-presidential nominee. Humphrey was the on the ticket when LBJ trounced Arizonan Barry Goldwater and his running mate, William Miller of New York. Two years later, Humphrey said: “To this day I have respect for Barry Goldwater and his running mate, what’s-his-name.”