Via the NRO web briefing:

Relative to past presidential nominees – Barack Obama has little relevant experience. His résumé is comparable to past “phenom” candidates Thomas Dewey and William Jennings Bryan. As a political matter, this means two things for Obama. First, as everybody knows, it is a direct weapon to use against him, which the McCain campaign has been doing for some time with its “Ready to Lead?” attacks.
Second, it means the definition of “Barack Obama” is more open to interpretation than other past nominees. The Obama campaign has used this vagueness to great effect. Simply put, because Obama has a slender record, he can be many things to many people. He can be the prophet of a new age to the chi tea crowd in Hyde Park, and a hardy Jacksonian fighter to the black coffee crowd in Youngstown. Politicians have been doing this dance routine for centuries. The fact that Obama’s story is hardly conditioned by a paper trail enables him to do this with more facility than most contemporary politicians.
But this does not mean that Obama “is” only who he says he is. His thin record is potentially a double-edged sword because anybody can try to define him. With the mentioning of William Ayers, the GOP has just now begun the process of offering its alternative definition of the junior senator from Illinois. It waited until October because, as I noted last week, anywhere between 20% and 30% of the electorate is now making up its mind. This is the time to begin this process.
What McCain and the Republicans will try to do is the opposite of what Obama and the Democrats are trying. The Democrats want to fold McCain into the generic Republican because they know that a generic Republican would be hard-pressed to do better than 45% this year. The Republicans, knowing that the country is in a mood to elect a generic Democrat, will speak specifically about Obama, trying to make him seem quite worse.