Barack Obama offers his most comprehensive response yet to the Bill Ayers controversy with this radio ad that hit the airwaves this morning, in at least the state of Wisconsin (a key battleground state.)

The Democratic presidential nominee’s ad correctly points out that William C. Ayers is an education professor, who served on an education-reform board with ties to GOP nominee John McCain. The ad reiterates that Ayers has no ties to Obama’s campaign. (Of course, the ad leaves out the “radical” part of Ayers’ past.)

This won’t be the end of it, though. McCain’s campaign had explicitly made it an issue through TV, web, and radio ads, but he didn’t bring up the Ayers controversy in the second presidential debate. After, Obama practically dared him to, essentially declaring that if McCain had something to say, he should say it to his face. McCain, who squares off against Obama in tomorrow night’s final debate, is now saying he’ll gladly oblige.