Everyone knows that Kerry has to do something to change the subject after the "foreign" leaders comment, falling down snowboarding and whatever other missteps he made recently. Having seen a few (too many) minutes of the bush rally today on cspan, I had an idea. I've heard bush use the phrase "I welcome that debate" several times, I'm sure it is a line in his standard stump speach.
The Kerry people could put together an ad (on the cheap) Starting with Kerry's challenge to debate bush followed by half a dozen clips of bush saying "I welcome that debate." At the end Kerry could re-challenge bush to monthly debates. It could be a way to go negative without going negative. It would certainly change the subject and likely put bush on the defensive for a few days.
Finally, the Kerry camp should steal a page from the BC playbook by only making a limited ad buy (WV?) and letting CNN and the other networks replay it over and over and over for free.