Pundits will spend all day chattering about what John McCain needs to do during tonight’s debate with Barack Obama, but they will agree on the broad principle: McCain needs to somehow engage Obama and attack him enough to seriously damage his credibility.

It will be McCain’s last opportunity to speak to a large audience—presumably, he doesn’t have the cash to compete with Obama’s primetime broadcast buy in two weeks—so he’ll have to make the most of it.

The worst-kept secret of how McCain will proceed tonight involves McCain’s all-but-assured line of attack involving Bill Ayers. Even our friends across the pond are talking about it.

We discussed Ayers here yesterday. And McCain was discussing Ayers yesterday, too—with a Chicago radio station, saying Obama’s earlier statement that McCain lacked the temerity to “say it to my face” “probably ensures” that McCain will make Obama’s relationship with the domestic terrorist a debate issue.

Count on a line from Obama about how he was 8 years old when Ayers was making all those nail bombs—

... Well, he’ll probably reword that last phrase.

The question is whether McCain will be able to fashion the Ayers issue into an effective weapon against Obama without inflicting any injuries upon himself.

Plan to join us at the live chat tonight and talk about it as it happens.