"To bring about the change this country needs, we're going to have to take on entrenched interests in Washington, D.C.," Edwards said in a Des Moines Register interview last week. "I think it's the difference between real change and trading one group of insiders for another group of insiders."

The debate is a sort of milestone in a campaign already eight months underway. Candidates have already headlined hundreds of campaign events around the state and unprecedented television advertising.

It caps an intense two week period of presidential campaigning in Iowa that began with a Republican debate in Des Moines on Aug. 5 and included the state GOP's high-profile straw poll in Ames on Aug 11.

Sunday 's debate gives Obama and Edwards the chance to revive campaigns that have stalled, Democratic strategist Donna Brazile said.

National polls show Obama trailing Clinton, with Edwards in third place.

Iowa is the only state where Edwards has had an advantage, with Clinton in striking distance and Obama typically in third place.

"Edwards and Obama are stalled — competitive but stalled," said Brazile.

"This is an opportunity for them to come forward with their second act. I think those two candidates are really going to try to distinguish themselves in terms of their message."

Obama, like Edwards, has begun sharpening his differences with Clinton, focusing on foreign policy in light of their different answers to a question during a South Carolina debate last month.

Obama said he would be willing to meet with the leaders of Cuba, Iran, North Korea, Syria and Venezuela in his first year in office and without diplomatic preconditions.

Clinton, who called such a promise "naive," said intense diplomatic spadework would have to come first, although she has said the U.S. should negotiate with its enemies.

Obama referred to Clinton 's position as "Bush-light," and has continued to associate her stance with the past.

Sunday 's debate will be another opportunity for Obama to set himself apart from not only President Bush, but the Washington establishment, including Clinton, Obama 's chief consultant David Axelrod said.

"The question is whether or not our fundamental problem is simply George Bush," Axelrod said. "Bush may have exacerbated it, but we've got this extraordinary polarity and divisiveness in our politics and the question is: Who is the most likely to change those things?"

Clinton plans Sunday to continue trying to portray herself as the most experienced while chipping away at those doubts about her electability, her campaign manager Howard Wolfson said.

"Our belief is the more people see of her, the more people like her," Wolfson said. "I think it 's to ensure that people are forming their impressions of her based on accurate information about her and a debate in Iowa is another important opportunity for people to see her directly."

Brazile said the debate will also be important for Biden, Dodd and Richardson, who trail the three better-known candidates in Iowa, but could receive a boost in their support if they have a memorable applause line during the debate.

Biden and Dodd, both longtime senators with foreign policy experience, have in recent weeks criticized the national security credentials of Obama, a first-term senator. Meanwhile, Richardson has advertised aggressively in Iowa, promoting his proposal to remove all U.S. troops from Iraq and touting his experience as a former congressman and U.S. energy secretary.

Richardson has recently begun polling regularly in fourth place in Iowa, while Biden and Dodd remain in the single digits.

One of the three could have an unexpectedly memorable performance, similar to Republican presidential candidate Mike Huckabee 's well-received speech and second-place finish at the straw poll, Brazile said.

"This could be a big moment for Biden or Dodd or Richardson," Brazile said.

"They could do what Huckabee did and take advantage of some of the weaknesses in the top-tier candidates."

Gravel and Kucinich have scant support and little or no campaign organization in Iowa.