Barack Obama and John McCain clashed Sunday over Iraq war policy and the economy as the Democratic party aimed a new attack against McCain with an advertisement questioning a remark he made over keeping troops in Iraq for up to 100 years.

In an interview that aired Sunday, Obama praised the top U.S. commander in Iraq, Gen. David Petraeus, for the progress he has made in taming sectarian violence there, but said he would set the goal of removing troops in Iraq even if Petraeus -- who was nominated this week to take charge of U.S. Central Command -- recommended against it.

"I think Petraeus has done a good tactical job in Iraq. I think as a practical matter, obviously, that's where most of the attention has been devoted from this administration over the last several years," Obama said on "FOX News Sunday."

Asked what he would do if Petraeus recommended against withdrawing troops from Iraq -- an Obama campaign pledge -- Obama said "it would be stupid" not to listen, but in the end, he would keep the goal of withdrawing troops.

"I will listen to General Petraeus given the experience that he has accumulated over the last several years. It would be stupid of me to ignore what he has to say. But it is my job as president, it would be my job as commander in chief, to set the mission, to make the strategic decisions in light of the problems that we're having in Afghanistan, in light of the problems that we are having in Pakistan, the fact that Al Qaeda is strengthening, as our national intelligence estimates have indicated, since 2001," Obama said.

He added: "What I would do is I would say -- what I will do is say, 'We have a new mission. It is my strategic assessment that we have to provide a time table to the Iraqi government. I want you to tell me how best to execute this new assignment, and I am happy to listen to the tactical considerations and any ideas you have, but what I will not do is to continue to let the Iraqi government off the hook and allow them to put our foreign policy on ice while they dither about making decisions about how they're going to cooperate with each other.' "

McCain's campaign fired back a statement over the exchange, saying Obama's statements were "a complete contradiction of his record."

"Are we to believe Senator Obama has the strength to take recommendations from General Patraeus, who has advised against immediate withdrawal, when Obama has repeatedly shown that he can't stand up to the left-wing interests in his party?

"Again, Senator Obama said he would raise taxes on hardworking families during an economic downturn, which makes it clear that he doesn't understand the American economy. His inconsistencies raised many more questions about Barack Obama's strength than they answered, and American families cannot afford leadership that can't deliver," McCain spokesman Tucker Bounds said, according to a campaign statement.

Later in the day, McCain took Obama to task over comments he made regarding taxes.

During the FOX interview, Obama criticized McCain, saying McCain switching his position from opposing President Bush's tax cuts to later supporting them.

"Somewhere along the line, you know, his conscience took flight because he was looking to get nominated for the Republican -- as the Republican nominee," Obama said.

McCain fired back: "I think that my record is very clear of having had a vigorous proposal for tax cuts, for spending restraint. My efforts at bringing about a restraint in growth of spending are very clear while Senator Obama has supported the spending increases and sought pork barrell projects ... causing spending to be completely out of control."

Focusing on Obama's support of raising taxes, including the capital gains tax, McCain said Obama "has no understanding of the economy and that he is totally insensitive to the hopes and dreams and ambitions of 100 million Americans who would be effected by his almost doubling of the capital gains tax, not to mention his desire to increase the cap on social security, which again would have a devastating tax increase effect on Americna families. He is out of touch with the economy."

Meanwhile, the Democratic National Committee's new ad leveled against McCain is part of a half-million-dollar, three-week national cable television campaign aimed at linking the Arizona senator to the policies of President Bush.

The ad set to begin airing Monday accuses McCain of wanting to remain in Iraq for "maybe 100" years, a link to a remark McCain made in January while campaigning in New Hampshire. The ad concludes, "If all he offers is more of the same is John McCain the right choice for America's future?"

Since then, McCain has repeatedly said he has no intention of extending the war into the next century, but would keep a U.S. military presence in Iraq much as the United States has in Germany, Japan and South Korea.

Democratic National Committee Chairman Howard Dean said Sunday that McCain is "wrong" and "out of step" with public sentiment on the war.

"We're not arguing that he's going to be in war for a hundred years. We don't think we ought to be in Iraq for a hundred years under any circumstances," Dean said on NBC's "Meet the Press."

"Think of the hundreds of billions of dollars that are being spent in Iraq which we need right here at home to preserve American jobs. ... Secondly, if Senator McCain believes that you can occupy a country like Iraq for a hundred years without having a long war and violence and troops being hurt and killed, I think Senator McCain is wrong," Dean said.

Citing a poll, Dean said 70 percent of Americans do not want troops to stay in IRaq under any circumstances, adding, "He is out of step with the American people."

The Democratic candidates have also acknowledged they would keep non-combat troops in Iraq to ensure its stability. But they have said they would begin withdrawing combat troops promptly upon becoming president, a step McCain has said would be precipitous.

McCain first uttered the comments about the 100 years in Iraq in response to a question during a January town hall meeting in New Hampshire, saying he'd be "fine" with a long-term U.S. commitment in Iraq.

"President Bush has talked about our staying for 50 years, maybe 100. We've been in Japan for 60 years, in South Korea for 50 years or so. That would be fine with me as long as Americans are not being injured or harmed or killed," McCain said at the time.

As recently as March, Obama linked the troop presence to ongoing war, but has since tailored the message to ongoing troop presence, versus continued fighting.

The DNC has been organizing a drumbeat against McCain at the state party level to coincide with McCain's travels across the country.

Meanwhile, Obama has become a Republican target. The North Carolina Republican Party aired an ad, over McCain's objections, that uses remarks by Obama's former pastor to portray Obama as too extreme. The ad points out that the two Democrats running for state governor have endorsed the Illinois senator.

Freedom's Watch, a conservative group, and the National Republican Congressional Committee are running ads in Louisiana criticizing Obama's health care proposal and linking him to a Louisiana congressional candidate.

The Associated Press contributed to this report.