Barack Obama attempted to shore up his liberal credentials Tuesday, pushing back against charges that he is shifting toward the center as John McCain's campaign continued to cast him as a flip-flopper.

Obama, speaking at a town hall meeting outside Atlanta, Ga., blamed recent criticism from "my friends on the left" and "some of the media" in part on cynicism that ascribes political motives to every move candidates make.

"This whole notion that I am, you know, shifting to the center or that I am flip-flopping or this and that and the other. The people who say this apparently haven't been listening to me," he said.

Obama said he is "no doubt progressive," and that even though he talks openly about faith and takes a few positions his supporters might not agree with, "the notion that somehow that's me trying to look like I'm more centered -- more centrist -- is just not true."

The presumptive Democratic presidential nominee was responding in part to the controversy he stirred last week when he suggested he might "refine" his Iraq positions after he visits the country this summer.

On Tuesday, Obama held to a previous pledge to withdraw all U.S. troops within 16 months of taking office.

"We're talking about common sense. We cannot be there forever," he said. "I am going to bring this war to an end."

In response, the John McCain campaign released a list of 17 alleged flip-flops, ranging from Iraq to campaign finance to trade to the recent Supreme Court decision invalidating the Washington, D.C., handgun ban.

"Since becoming his party's presumptive nominee, Barack Obama has changed numerous positions and will continue to change his stances as he puts politics ahead of principle," McCain spokesman Tucker Bounds said in a statement. "From growing our economy to securing our nation, Barack Obama has proven his rhetoric to be nothing but empty words and broken pledges that are at odds with his left-wing partisan record.

"Barack Obama is wrong: everyone's been listening and still nobody knows what Barack Obama truly believes."

Speaking on FOX News Tuesday morning, McCain stopped short of calling him an outright flip-flopper.

"I think there's definitely been shifts in position, and one of them is Iraq," McCain said. "And I'll be very interested in seeing what he has to say when he returns."

Since wrapping up the Democratic nomination last month, Obama has voiced positions that break with the Democratic Party's left and seem to shade his own past positions on a range of subjects. He's drawn criticism from some liberal Democrats who question his loyalty and from Republicans who accuse him of flip-flopping.

Originally best known as an anti-Iraq war candidate, his general election commercials appear nonpartisan and make an obvious play for voters across the political spectrum by focusing on family values and patriotism as well as issues like "welfare to work" and lower taxes.

Over the past few weeks, he has angered liberals by supporting compromise electronic surveillance rules for the government's wiretapping program even though the bill provided immunity, which he opposed last year, for telecommunications companies that conducted warrantless eavesdropping. When the Supreme Court overturned the District of Columbia's gun ban, he said he favors both an individual's right to bear firearms and a government's right to regulate them.

And, he broke with death penalty opponents when he disagreed with the Supreme Court's decision outlawing executions of people who rape children.

On Iraq, he has gone from a boisterous end-the-war call that endeared him to the left flank to more nuanced talk on the issue.

Obama on Tuesday raised the Supreme Court gun rights ruling.

"I actually have said that I agree with that for years, even before the ruling came down," he said. He said that doesn't contradict his view that "we've got decent controls over the use of illegal firearms in our community."

FOX News' James Rosen and The Associated Press contributed to this report.