The presidential campaign shifts from the roads of Pennsylvania and North Carolina to Capitol Hill on Tuesday when Gen. David Petraeus and and Ambassador Ryan Crocker testify on the Iraq war before the United States Senate. With television cameras catching every moment, the Q&As with Sen. John McCain, Sen. Barack Obama and Sen. Hillary Clinton should offer insight into the candidates' thinking on the war and how they would deal with those in charge.

McCain and Clinton will be asking questions as members of the Senate Armed Services Committee, where McCain is the most senior Republican. Obama will be asking questions at a separate Senate Foreign Relations Committee hearing scheduled later in the day.

Here's some pre-game handicapping:

• McCain scores points if he can make it clear that his support of the troop surge last year paid off with less violence in Iraq. He will be relentless in highlighting the positives of the Bush administration strategy -- and as a result, his own wisdom in supporting the president.
  
• No candidate has more on the line than Clinton. She needs to be the toughest, most intelligent questioner to bolster her argument that she will be "ready on day one" to become Commander in Chief. But she also needs to prove to Democrats voting in the next few states that she is committed to ending the war.
  
• Expect Obama to play it safe. He needs to come across as intelligent and ready, but it's more important that he make no mistakes that could cause Democratic superdelegates to re-evaluate him as a candidate. Having the stage to himself at a different committee hearing should be an advantage.