In 1995 Obama and Ayers really were both involved with the Chicago Annenberg Challenge—part of a national school reform effort financed by the publisher Walter Annenberg—along with various others, including the state's Republican governor. As it happens, Ayers’s and Obama’s relationship in this endeavor was no more than incidental. But suppose it had been more than that? Suppose Obama, a state legislator interested in urban problems, and Ayers, an education professor, had collaborated intensively on some local education project. What difference would it make?

The very same Times article observes that Chicago Mayor Richard Daley has "long consulted Mr. Ayers on school issues" and quotes him as saying that Ayers has "done a lot of good in this city and nationally." Whether that's a good thing or a bad thing should, surely, be determined by the quality of Ayers' advice about education policy rather than his views on whether or not a domestic bombing campaign was a morally acceptable response to the United States' wrongheaded prosecution of the Vietnam War. (For the record: it wasn't.)

If there were reason to believe that Obama harbored intentions of appointing Ayers to a national security post, or of using the powers of the presidency to orchestrate a bombing of the Pentagon, then there would be important questions to raise during a political campaign. But the idea that merely knowing somebody who has radical opinions ought to constitute a devastating objection to someone's political career is both wrongheaded and dangerous.

It’s wrongheaded because merely pointing out an association is lazy: it doesn’t do the harder work of establishing a connection between the relationship and Obama’s ability to govern. The McCain campaign has failed to do that.

And it’s dangerous because guilt by association can apply to just about anyone, and heading down that slippery slope would have perverse consequences. I have no idea what the vast majority of my friends think about the Weather Underground. I hope they have sound views, but if I found out otherwise I'd hate to have to stop hanging out with them. And, indeed, it seems to me that it would be a bit perverse to do so—so perverse that I trust nobody has any intention of actually trying to apply a guilt-by-association doctrine in any rigorous way.

Ayers is an extreme figure. But then again so is G. Gordon Liddy, the former White House "plumber" and Watergate burglar. On behalf of the Nixon administration he masterminded a break-in at the office of Daniel Ellsberg's psychiatrist and managed a 20-year prison sentence only because his most far-fetched schemes (including kidnapping anti-war protestors and bombing the Brookings Institution) never came to fruition. Liddy's sentence was commuted by Jimmy Carter, and since that time he's built a career as a radio host. McCain has appeared on Liddy's show and congratulated him for his "continued success and adherence to the principles and philosophies that keep our nation great." Are we supposed to hold McCain accountable for this association?