Obama should do this at his rallies/town halls if he's not already:

[Mayberry Machiavelli]

Ask for a show of hands for audience members who've had the McCain problem of not knowing how many homes they own.

Such a common problem after all.

Then follow up with comments on how the struggles of people to buy a home, or stay in theirs, contrasts with McCain's situation.

Good for a few video clips on TV coverage to keep the thing in circulation, up to a point.