Have him get snubbed like Jeffords was snubbed. Have Bush, whom McCain was still very bitter towards, organize Republican events and specifically exclude McCain. Shouldn't be very hard, Bush didn't feel any need to appease McCain till after 2004.
So, have McCain go turncoat. The Dems were courting him for it IOTL. Then, come 2004, he is a Republican-turned-Democrat and might see Kerry's offer as the only way to get a say in any future administration.