Many have seen talk of a proposed Kerry-McCain ticket, since they are longtime friends, and while McCain has been a loyal GOP member and endorsed the President, it's well know he is no real fan of Bush.
McCain has said he would not take the Democrat's VP nomination and no wonder. What's in it for him except a chance at the Vice Presidency? He would lose his Senate seat and probably never regain it. His old party would disown him and brand him a traitor. The Democrats would never nominate him for President when Kerry's term was over. Why trade Senator for a chance at VP and the end of your political career?
Another, much more radical suggestion comes to mind. McCain-Kerry. This seems like a ticket that would win. The only question is whether more Democrats would bolt from the movement by not voting, voting Nader or trying to run a backup Democrat (Edwards, Dean, etc.) than Republicans would switch to the bipartisan ticket of McCain & Kerry.
I think this ticket could win, and has the best chance of defeating Bush of any ticket that might happen. For those in the "Anybody but Bush in 2004" camp, this would be the reason to support it.
But would more ordinary democrats support it? The machinations required would be large but I believe workable. (Many delegates to the Dem Convention are already committed to vote for Kerry or another.)
McCain would have to agree to compromises, to agree to be a bipartisan President -- just Democratic enough to keep the Democrats on board, and Republican enough to win over the moderate Republicans under-thrilled with Bush, the ones he had supporting him against Bush last time. McCain is a man of principle, he would keep promises he made, I believe.
He would promise to do some Democrat items, and back off on Republican ones. (For example, abortion, supreme court appointments and a few other items.) He might even promise to resign at some point in his term, or only run one term.
McCain gets the brass ring -- he gets to be President, and get rid of Bush. That's worth risking your political career over. Kerry gets to be VP in a White House that promises to give a lot of duties to the VP, and he becomes the presumptive nominee for President after the McCain-Kerry administration is done.
The alternative is a 50% (probably less) chance of being President, and a 50% chance of 4 more years of GWB.