He's showing a lot more grace and class about this than many of his supporters.
As a candidate, he was weak. As a president, he would have been a disaster on the scale of Neville Chamberlain, perhaps even Vidkun Quisling. As a man, he barely qualifies as such. But for all that, he's a better human being than those who most badly wanted him to win. Howard Dean would have been more their style: a mouth-foaming moonbat filled with bitterness. But the mainstream Democrat voters weren't quite loony enough to nominate Dean.
Kerry is very much a man of the squishy left, who becomes a tool of the hard Left by virtue of his spinelessness and impressionability. His obedient service to North Vietnam may amount to treason, but that doesn't make him a Benedict Arnold. Arnold had more substance, and was able to understand what he was doing. Alcibiades, too, was more self-aware than Kerry. The people who use Kerry are serious, and they are very bad people. But Kerry himself is not serious. He's too shallow to appreciate the meaning of his actions. He's strictly a tool. And now he's a tool of questionable utility.
But at least he knows when he's lost the election. And he has enough good sense not to drag it out. He's better than Al Gore on that count.