Why is Kerry on the offensive?

Been reading Kos for a couple of weeks, as I've rapidly become addicted to following this race. Now I need to start contributing to others' addiction...

According to the AP, Kerry is now attacking Dean, saying he "flip-flops" on issues.

This strategy boggles my mind on several levels:

A. Who is Kerry to talk? He's flip-flopped on Iraq far more recently and dramatically than any of Dean's change of minds.

B. I thought the conventional wisdom after Iowa was that negative attacks don't work. Even Kerry's spokeswoman said that yesterday about Dean's attacks.

C. He's up by as much as 10 to 15 points in most polls. Why bother??

The only thing I can surmise is that he's going for the KO punch early because he's not looking for a long fight. Either that adds credence to the rumor that he really isn't set for the tons of primaries to come, or he thinks Dean is on the ropes, and wants to knock him out and focus on Edwards in the south.

Thoughts?