WASHINGTON (Reuters) - Democrat Barack Obama’s presidential campaign is signalling more flexibility on his pledge to quickly pull U.S. troops out of Iraq if elected as part of a move toward the political centre.

Obama’s emerging shift of nuance on Iraq, the signature issue that helped him defeat Democrat rival Hillary Clinton to win his party’s presidential nomination, comes as he prepares to make his first trip to Iraq.

The Illinois senator has repeatedly pledged to withdraw U.S. troops from Iraq, one brigade every month until all are out in 16 months. Last September he argued, “the best way to protect our security and to pressure Iraq’s leaders to resolve their civil war is to immediately begin to remove our combat troops.”

Aides say Obama is still committed to the 16-month goal but they appear to be leaving him wiggle room now that the U.S. troop surge is credited with bringing some stability there.

Susan Rice, a top Obama foreign policy adviser, told MSNBC on Tuesday that “we absolutely have to draw down and redeploy our forces from Iraq.”

“But he has said over and over again we have to be as careful getting out as George Bush was careless getting in. So he will redeploy our forces responsibly, at a rate that our commanders say is safe and sustainable.”

Letting commanders have a say in the pace of withdrawal is new language from the Obama campaign.

Anthony Lake, who was Democratic President Bill Clinton’s national security adviser and now a senior Obama foreign policy adviser, told the Financial Times Obama would maintain a “residual force for clearly defined missions” in Iraq.

This would include military training and “preparedness to go back in if there are specific acts of genocidal violence.”

Lake compared the Iraq war to the conflict in Vietnam in citing the need to leave behind a functioning Iraqi government.

“It is common sense that we could not leave Vietnam successfully unless we left behind a government in Saigon that could govern successfully,” he told the newspaper, lamenting that this view was not obvious enough to many U.S. politicians at the time.

Obama recently has been shifting toward more moderate positions on several key issues -- Republicans call it politically expedient flip-flopping -- now that he has won his party’s nomination and will face Republican John McCain in the November 4 election.

He abandoned a vow to reopen the North American Free Trade Agreement for renegotiations with Mexico and Canada, did not oppose a Supreme Court decision last week striking down Washington’s gun ban and has said he would support expanding the government’s wiretap authority.

LIBERALS TAKE NOTICE

The liberal left that helped propel Obama to the nomination is taking notice.

“I can unequivocally say: the Obama campaign is making a very serious mistake,” said Ariana Huffington, writing on the liberal Huffington Post blog. “Tacking to the centre is a losing strategy.”

Republicans, however, are sceptical that Obama, once considered the most liberal senator in Washington, is really becoming a centrist.

“Some of these things, he’s trying to look centrist,” said Republican strategist John Feehery. “But the fact is, he’s going to go hard left.”

McCain is a strong backer of current U.S. Iraq strategy. He has repeatedly pounded Obama for never having met with the top U.S. commander in Iraq, Gen. David Petraeus, who is to report to Congress again this month on the effects of the troop surge ordered by President George W. Bush in early 2007.

McCain has said he believes the Iraq war can be won by 2013, leaving a functioning democracy there and allowing most U.S. troops to come home.

A Time magazine poll last week said McCain leads Obama on the Iraq issue 48 percent to 38 percent, although 56 percent said they would like to see troops brought home within the next two years.

By moderating his Iraq pledges, Obama risks angering liberals frustrated by the inability of Democrats to get U.S. troops out of Iraq since winning control of Congress in 2006.

Democratic strategist Liz Chadderdon predicted Obama would talk less and less about a timeline for withdrawal but would not change his core position that U.S. troops must leave Iraq.

“If Obama completely reversed on getting out of Iraq, I do think the base would walk away,” she said. “I think he knows that and I think you’ll never hear him say that.”

But she said that in general, Democrats are willing to let Obama straddle some issues.

“We want to go back into the White House. And if that means we have to give him a longer leash on certain issues, we’re going to give him a leash,” she said.

(Editing by Alan Elsner)