John Kerry might not strike you as a wild and crazy guy, but he may be about to do something as nutty as it gets in politics: choose a Republican, John McCain, as his running mate.

The official story, of course, is that it's not going to happen. Kerry categorically refuses to talk about who is under consideration, and when asked, McCain has made remarks like "I have totally ruled it out" and "I will not be vice president of the United States."

Even so, the idea continues to percolate, largely because it offers the prospect of such an interesting (indeed, supporters say revolutionary) pairing. Kerry and McCain are longtime friends in the Senate, and as fellow Vietnam War heroes (McCain spent five-and-a-half years as a prisoner of war; Kerry was awarded three Purple Hearts) they both command respect on national security issues. With his reputation as a maverick and political straight shooter, McCain could help defuse the Republican characterization of Kerry as an indecisive waffler. Most importantly, choosing the popular McCain could energize voters and transform the race from the neck-and-neck fight that many predict into a Kerry landslide.

So what would be so wild and crazy about choosing McCain? Only that it violates every rule in the vice-president-selection handbook.

The tricky thing about picking a vice president is that everyone has a different theory of what to look for. Some say Kerry should pick a running mate from a region of the country that he hopes to win this fall, like the South (think Florida Senator Bill Nelson) or the Midwest (think Missouri's Dick Gephardt). Others preach the virtue of building on strengths (think fellow veteran Wesley Clark) or shoring up weaknesses (think the youthful John Edwards). Still others say Kerry should look to diversity and the potential to make history (think the Hispanic governor of New Mexico, Bill Richardson, or Arizona governor Janet Napolitano).

Those points, however, are far more debatable than these two hard-and-fast election-year rules: You don't pick a member of the opposite party, and you don't pick someone who will infuriate your party's base. In choosing McCain, Kerry might break both, because while independents may drool at the prospect of a Kerry/ McCain ticket, many hard-core Democrats are less than enthused. McCain is not only a Republican, he's often a conservative one: He is anti-abortion, pro-free trade and pro-gun.

Yet Kerry hopes that most Democrats are so anti-Bush that they will support him in November, no matter who the vice president is. Adding to that number could be the millions of voters who admire the independent McCain and love the prospect of a fusion ticket. A CBS poll released last week captures this appeal: While Kerry leads Bush by a margin of 49 percent to 41 percent, a Kerry/McCain ticket leads a Bush/Cheney ticket by the much larger margin of 53 percent to 39 percent. In addition, the poll shows McCain would help Kerry draw more independents, more Republicans and more young voters.

So is Kerry going to get all crazy? Actually, it may depend on McCain. Despite his independent streak, McCain is a fairly loyal Republican and, in fact, is chairman of President Bush's re-election campaign in Arizona. Yet it is no secret that he is still a little bitter over the 2000 Republican primaries that he lost to Bush in a hard-fought (and sometimes ugly) battle. Could the opportunity for payback -- coupled with McCain's maverick mojo -- be enough to convince him to abandon his party?

Perhaps more important is the question of whether McCain would be willing to do the main job of the vice president -- that is, play second banana to the president. Franklin Roosevelt's vice president, John Garner, once said, "The vice presidency isn't worth a bucket of warm spit," and for that meager reward Kerry would likely ask McCain to toe the Democratic party line on a variety of issues. Could that be too much for McCain to handle?

At this point, the only people who really know are both named John. But before Kerry and McCain make their final decisions, they may want to consider another factor: history. Only two times in American history has a split ticket won the White House: Abraham Lincoln/ Andrew Johnson in 1864, and William Henry Harrison/ John Tyler in 1840. Both presidents died while in office and the vice presidents took the reins -- and Congress tried to impeach them.