I don't know that McCain on the ticket would have helped Kerry that much. It would have given the GOP more ammunition for attacks and it would have caused problems with Kerry's base. Such a ticket might have worked with a moderate Republican.
It could have been Jesus Christ himself on the ticket and the Republicans were going to make attacks. I think the media would have fawned over McCain and given Kerry props for "bipartisanship." It might all be horseshit, but I believe it would have been very effective.
What problems with the base? Maybe Nader pulls more votes, but I don't know one Democrat who was EXCITED about Kerry. We all voted for him anyway. I don't think McCain would have made me less excited about a Kerry Presidency. McCain would have been a terrific coup.