Barack Obama, appearing to seek flexibility with his Iraq withdrawal policy, may have finally given Republicans ammunition they can use.

The Illinois senator has so far been impervious, at least in the polls, to the various labels John McCain and his supporters have thrown at him since he clinched the nomination a month ago.

They tried labeling him as a liberal, and an elitist, and then a pessimist, calling him "Dr. No" on energy issues. McCain has recently settled on the theme that Obama cannot be trusted, calling him out for reversing course on several topics.

But until Thursday, those positions were issues like public financing -- a topic unlikely to sway voters in large numbers in November.

With the difficulty Obama showed in explaining his Iraq withdrawal policy, he is dancing on a position that has been integral to his campaign.

Republicans seized on his comments Thursday, when he said he's willing to "refine" his Iraq policy. They are attempting to drive home the developing message that Obama will say anything for the sake of his political advantage.

"It is a flip-flop," Republican strategist Angela McGlowan told FOX News. "He's changing his tune to be more attractive to the whole country, and I think he's going to flip-flop on more things."

Unequivocal opposition to the Iraq war drove Obama's entrance into the presidential race. It helped him defeat Hillary Clinton for the nomination. It made him a darling of the anti-war activists who are now prominent and influential in the Democratic Party.

He repeatedly said during the primary that he would end the war in 2009, and that he would withdraw all U.S. brigades from Iraq within 16 months of taking office.

Obama called a second press conference Thursday to explain that he has always been consistent on the topic and that he was not talking about refining his withdrawal timeline. He said his upcoming trip to Iraq and the discussions he plans to hold with commanders on the ground will "inform" his plans, but that he intends to end the war as president.

"I have not equivocated on that position, I am not searching for maneuvering room with respect to that position," he said.

However, he again seemed to leave the door open for revision.

"If it turned out, for example, that we had to, in certain months slow the pace (of withdrawal) because of the safety of American troops ... of course we would take that into account," he said.

"I would be a poor commander in chief if I didn't take the facts on the ground into account."

It was not clear how Obama could slow the pace of withdrawal without extending his timetable.

Republicans portrayed the words as a prelude to, and even an admission of, a total reversal.

Conservative columnist Charles Krauthammer wrote, "the reversal is coming" and repeated a claim that "by Election Day Obama will have erased all meaningful differences with McCain on withdrawal from Iraq."

McCain spokesman Brian Rogers said in a statement that "Barack Obama has changed course and proven his past positions to be just empty words."

Rogers, sarcastically, congratulated Obama for accepting McCain's "principled stand" on Iraq.

And Republican National Committee spokesman Alex Conant said Iraq is "defining him as the self-interested, typical (politician) he really is."

The flip-flopper charge was a hallmark of the GOP campaign against 2004 Democratic nominee John Kerry.

Republicans branded as a "flip-flop" even the slightest rhetorical or policy change, and it was eventually successful.

Obama's perceived rhetorical shift was indeed slight Friday, but it was widely interpreted to have broader meaning.

Michael O'Hanlon, a senior fellow at the Brookings Institution and Iraq expert, said Obama was taking "contradictory" positions.

But Democrats said the Iraq statement was nowhere close to a flip-flop.

"He's not walking back, he's reaffirming his policy and basically saying that he's got to listen to the best military and strategic and national security advice that he can get," New Mexico Gov. Bill Richardson, an Obama supporter, told FOX News.

Democratic strategist Bob Beckel said Obama "said exactly what he should say."

"If he didn't do that, he would be jumped on as someone who's unwilling to change," Beckel told FOX News. "He still said he's gonna get these troops out within 16 months, the question is the pace of removal."

The Iraq flap comes as Obama tries openly to court voters in traditionally Republican states. He was in Montana Friday for the Fourth of July, after stopping in Colorado and North Dakota earlier in the week.

Both candidates are trying to court moderates and independents, and are modifying their post-primary message.

McCain has put extra emphasis on issues of energy independence and energy efficiency, and is distancing himself from President Bush.

Obama has taken positions recently that could appeal to moderates and Republicans.

He not only embraced but promised to expand Bush's program to give more anti-poverty grants to religious groups, a split with Democratic orthodoxy.

Obama also said "mental distress" should not count as a health exception that would permit a late-term abortion, saying "it has to be a serious physical issue," addressing a matter considered crucial to abortion rights activists.

His campaign announced that next week Obama will "continue his aggressive effort to expand the 2008 electoral battleground by campaigning in states that President Bush won during the general election in 2000 and 2004."

The Obama campaign heads next week to Georgia, North Carolina, Virginia and Ohio.

FOX News' Major Garrett and The Associated Press contributed to this report.