All three presidential contenders serve on Senate panels that will hear and question the top U.S. military commander in Iraq when he testifies Tuesday. McCain and Clinton serve on the Armed Services Committee; Obama is a member of the Foreign Relations Committee.
The backdrop to the Petraeus hearings includes the stark difference on the war between Republicans and Democrats, an increase in fighting in Iraq's southern region and a deeply disapproving public. Petraeus' status report not only will shape the administration's policy for the next several months, but also how the war will figure in the general election campaign.
With the presidential election just seven months off, President Bush's successor is certain to inherit the war that has cost more than 4,000 U.S. military lives and nearly $500 billion over five years.
"Overall, it's a remarkable success - overall with significant challenges ahead," McCain told The Associated Press recently as he predicted what Petraeus will say about the troop-increase strategy and what he says he believes himself - even though the year of the buildup was the bloodiest yet for U.S. troops.
Clinton and Obama offer a sharp contrast to the Republican nominee-in-waiting.
"It is time to end this war as quickly and responsibly as possible," Clinton, a New York senator, said last month and argued that the current strategy has not accomplished its goals because Iraqis have not reached a political reconciliation.
Obama, an Illinois senator, also wants a quick end to the war and said Friday: "We still don't have a good answer to the question posed by Sen. (John) Warner the last time Gen. Petraeus appeared: How has this effort in Iraq made us safer and how do we expect it will make us safer in the long run?"
Obama said Bush and McCain have been "trumpeting improvements from a horrific situation to a simply unsustainable and intolerable situation."
Petraeus is expected to report that impressive security gains and modest political progress has occurred since the influx of U.S. troops, and that force withdrawals can continue this fall if officials conclude security would not be compromised. Even before he testifies, Republicans are seizing on signs of progress, while Democrats are highlighting deficiencies.
Public opinion on the war bodes poorly for McCain.
Like other surveys, a CBS News poll last month found that a majority of the country doesn't believe the United States should have ever gone to war in Iraq. More than half of the public thinks that the war is going badly and that the troop-increase strategy hasn't made the situation better. Two-thirds of the country is willing to have large numbers of U.S. troops in Iraq for two years or less, while nearly half say a significant level of forces would be acceptable only for another year.
Underscoring public uncertainty over who is best to lead on the issue, more than half of the country - 56 percent - said they were very or somewhat confident in McCain's ability to make the right decisions about Iraq - but Clinton and Obama got essentially the same marks.
Closely linked to the war, McCain resolutely backs the current strategy and wants to give the U.S. military more time to accomplish its goals. He insists he will take his cues from Petraeus - who he regards as one of the greatest generals in the country's history - even if doing so damages his campaign. The latest security challenges in Basra and the news that some Iraqi soldiers and police mutinied or refused to fight militants threaten to undercut him. Even so, the Arizona senator is certain to use his committee perch and Petraeus' appearance to embrace security gains over the past year as well as highlight any political and economic advances to argue that the strategy is working and the United States should not shift course. He will press those positions a day earlier in a speech to the Veterans of Foreign Wars in Kansas City, Mo.
Obama and Clinton, for their part, are expected to focus on what they view as slow-to-take political and economic changes in Iraq to press their case for a new direction. As they have elsewhere, they are likely to highlight the thousands of lives lost and billions of dollars spent in a war they argue was mismanaged from the start and diverted U.S. attention from the global hunt for terrorists.
At the same time, they probably will claim that hotspots like Basra provide evidence the situation isn't as good as it may seem, and argue that public sentiments run counter to the current strategy. Above all, the two are certain to press Petraeus on the plan ahead - and the precise conditions the next president will face.
