Click here to listen to "Ayers."

After several weeks of intense criticism from the right over his relationship with former 1960's radical William Ayers, Barack Obama is addressing the topic head-on in an ad.

A new radio spot (subscription) running in key states echoes a TV ad released by the campaign last Friday which criticized John McCain for his negative campaigning. But while the TV spot made only a veiled reference to Ayers, the announcer of the radio ad gets into the subject in some detail.

Introducing Ayers as a "professor of education," the announcer explains that he "once served with Obama on a school reform board, a board funded by conservative Republicans with ties to McCain." The spot reiterates a point that Obama camp has made repeatedly: that the Illinois senator was eight years old when Ayers began his radical activities as a member of the Weather Underground. "Ayers has had no role in Obama’s campaign and will have no role in his administration," the announcer says.

McCain opted not to bring Obama's relationship with Ayers up in the second presidential debate, despite the fact that he and running mate Sarah Palin were pounding the message on the campaign trail. The Arizona senator recently hinted in a radio interview, however, that the subject might surface in tonight's final showdown.