(Updates with Democratic candidates comment at debate)

By Andrea Shalal-Esa

WASHINGTON, Sept 9 (Reuters) - Top U.S. officials will likely warn Congress on Monday against major changes in U.S. policy in Iraq, but leading Democrats said that strategy had failed and it was time to think about getting U.S. troops out.

Gen. David Petraeus, the top general in Iraq, and U.S. Ambassador Ryan Crocker were expected to argue that a major decrease in U.S. troop levels could undercut any progress made under the "surge" of forces into Iraq this year, a U.S. official told Reuters on Sunday.

U.S. President George W. Bush, under mounting pressure to change course in Iraq, urged Democratic lawmakers demanding a timetable for troop withdrawal to listen to Petraeus and Crocker before "jumping to any conclusions."

Bush plans a prime-time speech on Thursday on the U.S. role in Iraq but was not expected to announce a major policy shift after four years of a deeply unpopular war that has claimed the lives of more than 3,700 U.S. troops and over 70,000 Iraqis.

The U.S. official, who asked not to be identified, said Petraeus and Crocker also were not expected to advocate any change in Iraq's leadership despite frustration at its failure to reconcile Iraq's Shi'ite, Sunni and Kurdish populations.

The Bush administration boosted troop levels -- now at 168,000 -- to improve security and allow Iraqi politicians time to stabilize the country.

The buildup has not done either, said Sen. Joe Biden of Delaware, a Democratic presidential candidate.

"The truth of the matter is that the American administration's policy and the surge are a failure," Biden told NBC.

He signaled that Petraeus and Crocker would face tough questioning this week when they testify before four congressional committees, including Biden's Senate Foreign Relations Committee.

'DEAD, FLAT WRONG'

In a letter to the troops on Friday, Petraeus said the Iraqi government's progress toward national reconciliation was disappointing, but cited some progress in establishing security.

"I think he's dead, flat wrong," Biden said. He said he would continue to insist on a target date to withdraw U.S. troops from Iraq.

At a debate at the University of Miami on Sunday night, the Democratic presidential candidates said they were skeptical of the report and repeated their calls for a speedy withdrawal of U.S. troops from Iraq.

"What I'm concerned about, about the Petraeus report, is that it will be basically a sales job by the White House, that it'll be a PR document -- because that's what we've continually gotten," said John Edwards, the 2004 Democratic vice presidential nominee.

Sen. Hillary Clinton of New York said she was against the "surge" when it was proposed and did not believe anything Petraeus or Crocker would tell Congress would "in any way undermine the basic problem: There is no military solution."

The Democrats won control of both houses of Congress last year largely due to anti-war sentiment, but they do not appear to have the votes to force a curtailment of the U.S. effort.

A majority of Americans say the United States made a mistake getting involved in the war in Iraq, and the recent troop buildup had either made things worse or had no impact at all, according to a New York Times/CBS News poll released on Sunday.

Still, 35 percent now say the buildup has helped improve the situation, up from 29 percent in May.

Sen. John McCain of Arizona, a Republican presidential candidate, insisted the troop buildup was starting to work.

"This new strategy has only been in effect for a relatively short period of time," McCain said on ABC. "I want us out too. But I want us out with honor." (Additional reporting by Arshad Mohammed, Nancy Waitz and John Whitesides)

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