Interesting notion. Perhaps the adds in South Carolina in 2000 are even more vicious? Perhaps Bush makes some gaffe that McCain takes as an insult?
Hmm, given Liberman's showing in 2006, I bet McCain keeps his seat. However, the fusion ticket with Kerry might have potential as well. I think McCain has the choice between defending his Senate seat to vindicate his choice to reject the Republicans and joining Kerry in order to satisfy potential Presidential ambitions.
A lot depends on just how Kerry makes the offer. Indeed, before we even get to 2004, there's the potential political fallout to consider. If McCain is vigorously decalring his opposition to Bush policy in 2002, then the events of 2003 might be different. It's hard to see McCain actually opposing the war resolution, but that's not the whole story. If McCain bolts in 2002, then he's effecitvely saying that you can oppose the Bush Republicans without being un-patriotic after 9/11. That has the potential to throw a big loop in the atmosphere of Capitol Hill.
Another big quetion is who McCain choses to Caucus with. He wouldn't change the control of the Senate. Would he chose to Caucus with the Democrats, like Jeffords did, in order to retain a Senate chairmanship? Or Would he chose to be a genuine independent and simply use his privileges as a Senator to make proposals on the floor?
Also, I wonder if McCain might attempt to resign to force a special election and run as his own replacement. Arizona's Constitution does require such in the event of a vacancy. This might be an attempt to vindicate his choice and demonstrate his continued appeal to voters. If he does this, though, he's declaring war on the Bush Administration and will make far greater waves. It would also be a big cost and he'd still need to run again in 2004.
All of this doesn't preclude McCain from accepting Kerry's offer (indeed, it sets him up to do so), but would the offer still exist? Would Kerry still win the Democratic primary? I would guess so, since the anti-war enthusiasm of the Dean supporters might be lessened if there's more bipartisan dissent against Bush. I could see Kerry getting pressure to keep the ticket "in the family" since the DNC would see the chance to re-take the White House all the more potently: there's no need for McCain's appeal unless Kerry insists on it.
On the other hand, there's no real reason to think that McCain's defection would really be interpreted in so harsh a light. Accordingly, he might feel an even greater weight not to accept Kerry's offer in order not to seem like a complete turncoat. Furthermore, McCain might want to fight for his seat in 2004 to vindicate his choice.