The John McCain campaign got a great deal when they rented a hall from the city of Homewood, Alabama for a fund raiser. They were charged 80 percent less than Democrats had been for the same space and given free convict labor from the local jail to do the set-up. The deal was probably completely illegal since Homewood Mayor Barry McCulley gave the McCain campaign their discount on his own authority and by bypassing the normal procedures. This could easily be regarded as an in-kind donation to a political candidate.
Let's give McCain himself the benefit of the doubt and assume he knew nothing about this. He needs to step forward and offer to pay the full rental fee right now before it's used against him. Even that might not be enough to avoid a fine from the FEC for accepting an illegal contribution, but it would give him a ethically defensible position before the voters. This should be a gift to the Democrats if they are on the ball. But there's only a 50-50 chance of that--at best.