First off, Sen. Kerry is going to roll in to 2008 as one of the few recognizable Presidential candidates, other than perhaps Sen. Hillary Clinton. Who are the Republicans going to field? Who knows, but it's not going to be Vice President Cheney. Furthermore, you're likely not going to see a Republican governor emerge as a successful Presidential candidate that fights the "powers that be" like Bill Clinton or Jimmy Carter did against long periods of Republican White House control (such a candidate is part of the "powers that be" to begin with). As for Republican candidates from the Congress , the Military or a previous administration's cabinet, Usedcarsalesman has to ask, "who do you know in that field who has something to fight for?"
Then we have the Democratic field: basically Senator John Kerry, Senator Hillary Clinton and her spouse and various lesser known Democratic politicians. In that pool of candidates, Usedcarsalesman give the nomination victory (and the Presidency) to Kerry. Kerry has already been through the Presidential Campaign ringer once and he did extremely well versus a sitting President who was in charge of a fearful country that had been attacked by a foreign entity. Kerry has lost other political races before, such as a congressional race in the 1970s; but, he came back from this defeat and won a Senate seat. So, Kerry has shown that he can rebound in to a stronger position -- he can run or President again after losing and win.
Usedcarsalesman has personally seen people who were not successful on a first attempt at a life-defining event succeed on their second, third or fourth attempt. One of the reasons for this follow up success is that the people -- the voters in Kerry's case -- who regulate the achievement of political success actually "respect" (a word Kerry used a few times in 2004) a candidate who keeps coming back for more after losses (think John Elway only leading Broncos to victory in the Superbowl on the 4th effort). Furthermore, Usedcarsalesman has even seen people who were not allies actually begin to extend grudging favors to people on a come-back trail; this all has to do with their respect for the will it takes to pursue the same thing again after a defeat.