Republicans are still making the argument that Sarah Palin has the necessary experience to serve as vice president she has spent less than two years as governor of Alaska--er, sorry, I mean Commander-in-Chief of the Alaska National Guard. After careful consideration, I've decided not to rebut this argument, lest I lend it even a shred of credibilty.

Instead, I'd like to dwell on why experience matters in a vice presidential candidate, perhaps even more than it matters in a presidential candidate. Nate Silver made one important argument here. When a president cannot serve out his or her term, whether because of incapacity, scandal, or death, it is, almost by definition, a crisis. As Nate notes, frequently "a president takes the Oath of Office under relatively calm waters, allowing them something of a learning curve." A crisis can stil present itself quickly--and, lord knows, presidents of both parties have made rookie mistakes for which the country paid dearly. But the margin for error would seem even slimmer when a vice president assumes power.

The other reason is the timing of modern campagins. Today, voters usually have more than a year to scrutinize the presidential candidates--to figure out what they believe and how they operate--before pulling the lever in November. But they don't get the same chance to see the vice persidential candidate in action. So it's particularly important vice presidents be known quantities--somebody whose record on the issues is clear and whose ability to lead is well-established.

Joe Biden fits this definition perfectly. He's got six terms in the Senate, including service as chairman of two high-profile committees, plus two runs at the presidency by which to judge him.

And Palin? This is the first time most Americans have even heard of her. Her political identity, such that it is, remains a mystery even to most of the political class. It seems unlikely that the McCain campaign will expose her to much press scrutiny--and, with just two months to go before Election Day, there won't be much chance even to see her campaign.

Put aside ideology for a moment. How on earth are voters supposed to make an informed judgment here? And what does it say about McCain's fitness for office that he'd ask the public to make such a leap of faith?

--Jonathan Cohn