I don't see Kerry being the VP candidate here. He was the party leader the cycle before and no way he'd be second fiddle the next election. I can see him leading the Dems in Congress or being Sec of State though. See Clark picking either Obama, Tim Kane, or Evan Bayh. Hillary would prob just be a supporter but I could see her easily being SOS or in Congress like in RL. Obama would give him charisma or star power, but I can see him picking Bayh too. But for this sake I'll keep Kerry in this scenario.
I say it would be a close race for most of the spring and early to mid summer. Clark does well with the Democratic base and moderates while Romney has the Republican base behind him and is fighting for the Indys. The Dems focus on the anti war effort, and attack Bush. Clark is modeled to look like an Ike esque candidate. People eat that up while Romney is portrayed as a Mr Burns type figure and scares people away. Clark has a least a 5 point lead until the conventions finish.
Then the economy collapses and Clark takes off. I don't see Romney making the "fundamentals of the economy is strong," gaffe or having Palin around but his gimmick and persona is as toxic. He behaves like he did in 2012 but struggles to attract the moderates and Indys due to his desire to cut more government. Clark meanwhile focuses on improving life domestically and ending the war. The Ike type persona calms the nation. I say Clark wins all 3 debates (especially the 3rd where Romney embarrasses himself over foreign policy,) but the VP debate's a draw and Jindal does surprisingly well. Romney hits the negative side of the plate like McCain in RL but Clark rebuffs the attacks and the public grows tired of it.
Clark/Kerry-380Romney/Jindal-158Clark has the lead solid after the final debate and by election day his victory was guaranteed minus a couple of states. Clark demolishes Romney nationwide and runs deep in the south picking up 5 states that were solidly for Dubya. He barely lost Indiana and Tennessee as Romney pulls them off thanks to heavy campaigning there.