And he has staked a lot on Sarah Palin.

Voters are unlikely to learn much that is new or surprising about Obama, McCain or Joe Biden over the next two months. Palin’s performance as the vice-presidential nominee, on the other hand, is the open and unresolved question of this campaign. She is, in a way, now the central figure in this fall’s electoral drama.

If Palin turns out not be up to the challenge for which McCain has selected her, McCain will pay a heavy price. His judgment about the most important choice he’s had to make this year will have been proved wanting. He won’t be able to plead that being right about the surge in Iraq should be judged as more important than being right about his vice-presidential pick.

McCain has gambled boldly on Palin. If she flops, McCain could lose by a landslide.

On the other hand, if Palin exceeds expectations, and her selection ends up looking both bold and wise, McCain could win.

The Palin pick already, as Noemie Emery wrote, “Wipes out the image of McCain as the crotchety elder and brings back that of the fly-boy and gambler, which is much more appealing, and the genuine person.” But of course McCain needs Palin to do well to prove he’s a shrewd and prescient gambler.

I spent an afternoon with Palin a little over a year ago in Juneau, and have followed her career pretty closely ever since. I think she can pull it off. I’m not the only one. The day after the V.P. announcement, I spoke with an old friend, James Muller, chairman of the political science department at the University of Alaska, Anchorage. He said that Palin “has been underestimated over and over again. She took on the party and state establishments here in Alaska, and left them reeling. She’s a very good campaigner, a quick study and a fighter.”

Muller called particular attention to her successes in passing an increase to the oil production tax and facilitating the future construction of a huge natural gas pipeline. “At first the oil companies thought she was naïve, and they’d have their way. Instead she faced them down and forced them to compromise on her terms.”

Can she face down the Democrats, Joe Biden and the national media over the next couple of months?

John McCain is betting she can. Perhaps, as he pondered his vice-presidential selection, he recalled the advice of Margaret Thatcher: “In politics if you want anything said, ask a man. If you want anything done, ask a woman.”