John McCain needs to pick a running mate. He has a tough decision to make.

A very old presidential candidate in a party that has lost the confidence of the American people, McCain will likely face a young, charismatic black guy who promises to "turn the page" away from the Clinton-Bush era, toward a new era of bipartisanship and unity. Meanwhile, the base of McCain's party has serious and legitimate misgivings about him.

Those misgivings have prompted some on the right, including my National Review colleagues Ramesh Ponnuru and Kate O'Beirne, to argue that McCain should offer a one-term-and-out pledge. This not only would defuse the age issue but would telegraph in concrete terms that McCain really is a different kind of politician (while Obama only pretends to be). "The public, never fond of Washington politicians as a class, is especially sick of them these days," writes Ponnuru. "It longs for leaders who are above the poisonous partisanship they see on TV. A one-term pledge would remind people that McCain has been such a leader."

But at the same time, conservatives like Ponnuru argue that McCain needs to pick a running mate who will reassure conservatives. Indeed, National Review has editorialized that delegates to the GOP convention should revolt if McCain picks a pro-choice or otherwise squishy veep candidate. "In picking a running mate Senator McCain will also be conferring front-runner status on a candidate for his party's future nomination. A selection that reassures wary conservatives will help to enthuse his supporters for the tough race he faces," opined my colleagues.

This makes consummate sense - if McCain picks a Republican. But by picking a staunch-conservative Republican, McCain would undercut his standing with the independents and swing voters he needs to win. If he picks a squishy Republican, many conservative voters will stay home in disgust.

But what if he picks a Democrat? Specifically, what if he picks a Democrat while pledging one term and out?

As the parties have become less coalitional and more ideological, the No. 2 slot on the presidential ticket is increasingly seen as an opportunity for national marketing rather than regional deal-making. Bill Clinton picked Al Gore not to win a specific state or constituency, but to shore up Clinton's image as a youthful, moderate reformer. George W. Bush selected Dick Cheney for any number of reasons (though, contrary to rumor, none of them have to do with Cheney making Bush an offer he couldn't refuse on a hunting trip), but capturing Wyoming's three electoral votes wasn't one of them.

With the exception of Louisiana Gov. Bobby Jindal (and possibly Alaska Gov. Sarah Palin), it's difficult to see how any of the advertised picks for the veep slot help McCain sell himself as a different kind of Republican. And, at age 36, the still-green Jindal might not only seem like a gimmick, but he would undermine McCain on two fronts: He'd remind voters of McCain's age, and he'd diminish the anti-Obama argument that experience, particularly in foreign policy, really matters.

Meanwhile, a national-unity ticket would, among other things, expose Obama's fraudulent claims to be a post-partisan uniter and reformer. The party-line, left-wing Democrat has done almost nothing in his short political career to support either claim. He is a product of the profoundly corrupt Chicago machine, not an enemy of it. And his definition of bipartisanship amounts to welcoming the unqualified support of Republicans who support his liberal agenda. The most liberal member of the Senate in 2007, according to National Journal, wasn't even a member of the bipartisan gang of 14.

Such a daring move on McCain's part would also signal that the country might enjoy a timeout from partisan rancor. Even the Obama-sycophantic mainstream press would have to admire such a profound gesture.

The benefit for Republicans might be substantial. The party could rightly claim to have the bigger tent and the stronger commitment to serious reform. And for movement conservatives, the next four years could be a time for much-needed rebuilding. Obviously, a Joe Lieberman or Sam Nunn would not be the presumptive front-runner for the GOP nomination in 2012. And the lack of an heir apparent would encourage a healthy and vigorous debate for the future of the party.

McCain would still have to reassure Republicans that he would be reliable on judges and other issues vital to conservatives. But a unity ticket would provide the greatest assurance of them all: Barack Obama wouldn't be the one picking judges.