Let it be stated at the outset that I will support ANY democrat who wins the nomination, and I think all 6 mainstream candidates CAN defeat Bush w/ the right campaign. As such, I will refrain from criticizing any of them or saying that they cannot win.
That said, I support Kerry because I think he'd have the BEST shot at beating Bush for the reason that he can truly unite the party. Securing the base is extremely important -- theoretically, it's possible to lose some of the base and make it up by winning more swing voters, but in reality it's very hard. A candidate who the base does not like is not going to have a great campaign. There's going to be a lack of enthusiasm and fire, possible defections, and many will stay home or badmouth the nominee throughout the fall, helping Bush.
Kerry is the BEST placed candidate to unite the party. He's acceptable to all the major groups within the party and he's sufficiently mainstream to compete well for swing voters and independents.
Though Kerry is currently not the hot name -- obviously, Dean is, let's look at who says they're willing to support Kerry: 75% of the move-on primary voters, who, despite voting overwhelmingly for Dean and Kucinich, agreed that they could well live w/ Kerry.
Kerry is perfectly acceptable to the vast majority of Deanies, because of his liberal positions and his outspokenness against Bush (less than Dean, but still quite a lot). He's acceptable to left-liberals. He's acceptable to upscale swing voters b/c of his moderation on economic issues. He's acceptable to labor b/c of his strong record in that area. He's acceptable to the centrists.
With Kerry there won't be the different groups in the party bad-mouthing his candidacy and hampering his run. None of the other candidates can say that. Edwards may come closest, but he's probably weaker in a general election b/c of his relative inexperience. Gephardt unites many, but alienates many in the party either for his strong prowar stance or for his protectionism on trade. Lieberman holds both the downscale swing voters and the upscale swing voters but loses the liberals and progressives who detest him. Dean earns the ire of the DLC and the centrists, who, despite all those on this site who despise them, ARE needed to win the general election.
Kerry can hold all those groups and make them all sufficiently excited to win in November '04. I'm not saying that no other candidate can win -- I despise it when people dismiss any of the mainstream 6 that way, b/c they all have a shot at the nomination and they all CAN win. But it'd be much more difficult for them to unite our fractious party.
If we are divided we will fall. Echoing others on the site, we need to unite and presented a common front against Bush. I think Kerry's the best placed to do that.
Check out this American Prospect link that says much of the same thing:
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http://www.prospect.org/webfeatures/2003/07/meyerson-h-07-24.html