By Randall Mikkelsen

WASHINGTON, Aug 24 (Reuters) - A Republican senator's call to begin a troop withdrawal from Iraq is a wake-up call for President George W. Bush as he fights to keep his policy from unraveling, but it may not immediately sway votes in Congress for a pullout.

Sen. John Warner of Virginia, an influential congressional voice on military affairs in Bush's Republican party, said on Thursday the president should announce next month an initial pullout of 5,000 troops who would be home by December.

Warner said it would help spur Iraqi Prime Minister Nuri al-Maliki's government to take long-delayed steps toward political reconciliation. But the call was also aimed at catching Bush's attention, said Brookings Institution congressional analyst Thomas Mann.

"The main thing he's trying to do is send a signal to the administration that they've got to begin taking some actions that show ... we're not just going to stay there (in Iraq) indefinitely," Mann said.

The statement added to the pressure for change in Bush's Iraq policy, which is growing on many fronts.

"It's going to further isolate the president," said Iraq analyst Frederick Barton of the Center for Strategic and International Studies. "You're getting close to the point where something's got to happen."

A U.S. intelligence report on Thursday said sectarian violence would remain high in Iraq and the position of Maliki's government would get "more precarious," despite improvements in security.

The top U.S. military officer, departing Joint Chiefs of Staff Chairman Peter Pace, is expected to urge Bush to slash U.S. troop levels in Iraq next year, the Los Angeles Times reported on Friday. Pace, through a spokesman, called the article "speculative."

Leading Democrats in the congressional majority have called for Maliki's replacement, and the party is gearing up for a new round of troop-withdrawal initiatives when lawmakers return in September from a summer break. A major assessment on Iraq ordered by Congress is due Sept. 15 and is widely regarded as a potential trigger for a change in Iraq policy.

The unpopular war has featured prominently in the campaign for the November 2008 presidential election, with Democrats and some Republicans calling for a full withdrawal.

But Warner stopped short of making a break that would draw more Republican votes to mandate a pullout, an aide to the Senate's Democratic leadership said. Warner said on Thursday he still opposed Congress setting a withdrawal timetable.

"Recommendations for the president are far short of what is needed," said the aide. "The real question is, how is he going to vote?"

Rep. Peter Hoekstra of Michigan, the senior Republican on the House of Representatives Intelligence Committee, said Bush's goal of instilling democracy in Iraq "should not be our objective" and Iraq needed new leadership committed to fighting insurgents.

But he also opposed Warner's call for a withdrawal timetable, during an appearance on Michigan Public Television.

White House spokesman Gordon Johndroe disagreed that Warner's statement was a sign of waning support for Bush. "I don't think that Sen. Warner's position on Iraq has substantially changed. I'm not aware of any announcements of anyone changing their votes," he said.

The attention given to Warner's call and the intelligence report on Iraq has, however, drowned out a new push launched by Bush to rally support on Iraq by citing gains of a military escalation he launched this year and casting a democratic Iraq as vital to Middle East security.

A group of Bush administration supporters and former employees has launched a campaign in support of the Iraq war. The group, Freedom's Watch, said on its Web site it wanted to counter war opponents "who have dominated the public debate" in recent years.

But Barton said opposition to the war is becoming overwhelming. "I'd say we're close to the avalanche stage," he said. "The American exit from Iraq is inevitable, but the discussion is when and how and how to mitigate any negative effects."

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