It's just really stupid political strategy. And I'll tell you why.
Right now the media and several of the other campaigns are on the warpath against the Dean campaign, using everything they can dig up against him. Kerry has hurt himself by going so negative in NH. Clark, as the only other big alternative to Dean in NH, had brilliant success in using a positive campaign to pull soft Kerry and Dean support to him in NH, and by staying positive put himself in an unassailable position should Dean attempt to go negative.
This put Dean into a tight spot, unable to attack Clark without seeming like he was hearing footsteps, and also unable to attack Clark without seeming like he was the one going negative first. So Dean attempted a bit of minor negativity, and it bombed utterly. It's results were far out of proportion to what was done, (the flyering of a couple town hall meetings with relatively tame negative contextless soundbites) angering Dean supporters who didn't want to go negative on someone who had by and large left them alone, and making Clark look really good for staying above the fray and worrying Dean enough to be attacked.
Clark would have had to go negative sooner or later, simply because they're going to be the last two dogs in this fight and he won't be able to count on Kerry or Gephardt or Lieberman giving the negative attacks on Dean so that Clark doesn't have to. But he had at least 2 more weeks of negativity before he'd have to do that (after feb 3rd).
Now, with the Lehane link to negative media stories, Clark is suddenly vulnerable to attacks by the Dean campaign. He loses his mantle of high minded politician, and now Dean can attack him as hard as he attacks his other opponents. And the Deaniacs will buy it, because they'll feel like they were attacked as well.
It was a calculated risk for Clark, one that worked fairly well for a couple of weeks. But with 10 days to go before NH, the gambit fell a bit short of it's goal, and now he's going to have to deal with the fallout.
DISCLAIMER: Lehane has NOT been definitively linked. but there's a fair amount of speculation about it. Until Lehane is completely linked to the oppo that has made it to the press hounds, don't pass (too much) judgement.