The Democratic presidential contenders went on the attack Tuesday against front-runner Hillary Clinton, accusing her of shifting her positions for political convenience and saber-rattling against Iran.With Clinton holding a huge lead among Democrats in national opinion polls ahead of the November 2008 election, rivals Barack Obama, John Edwards, Chris Dodd, Bill Richardson and Joseph Biden all took turns aiming shots at the New York senator in hopes of bringing her back to the pack.Obama, who is running a distant second in polls to Clinton, said she had changed positions on trade, Iraq and Iran "whenever it's politically convenient."Edwards said she had flip-flopped on Social Security and Iraq, and helped President George W. Bush set the stage for war with Iran by voting for a Senate resolution labeling an Iranian military unit to be a terrorist group."I think the American people, given this historic moment in our country's history, deserve a president of the United States that they know will tell them the truth, and won't say one thing one time and something different at a different time," he said.Clinton defended her record in the Senate and the campaign trail as consistent.