Several of the Democratic presidential candidates have struggled with the Iraq issue because they are on record as having voted for the bill that permitted the invasion (the issue seems to have flummoxed Clark as well). Dean has made hay with this.
Take Senator Kerry as an example. I think he would probably be a fine President (as would several other of the Democratic hopefuls). He started his campaign as the notional front runner, and surrounded himself with consultants from the Democratic party establishment. And they ran the standard play from the Democratic playbook: if you are ahead, be cautious. Don't blow it. That might have worked against Gephardt, a very studied sort of populist, but it doesn't work against genuine populist insurgents (Dean, maybe Clark).
Caution is not such a terrible trait in a President, at least most of the time, but it proved a wasting strategy for the Kerry campaign. So, at long, long last, John Kerry has finally decided to attack Bush in his foreign policy soft underbelly. While I think that it's a good decision, it does make one wonder about the more general question of why so many of the candidates have avoided the obvious explanation for their vote: Bush lied to us.
