The ads tie McCain to President Bush and cite McCain's comments that there could be an American military presence in Iraq for 100 years. They are being run by the Democratic National Committee and the liberal group MoveOn.org.
"One hundred years in Iraq? And you thought no one could be worse than George Bush," an announcer says in the most recent ad, run by MoveOn.org.
McCain brought up the commercials at a town hall meeting Friday in Denver, saying they are lies. He doesn't deny saying "100 years" in connection with U.S. military operations in Iraq, but says he was clearly referring to a possible peacekeeping force - not a centurylong war, as critics imply.
"You have seen an ad campaign that is mounted against me that says I wanted to stay and fight in Iraq and fight for 100 years," McCain told about 300 people at the Robert E. Loup Jewish Community Center.
"My friends, it's a direct falsification, and I'm sorry that political campaigns have to deteriorate in this fashion," McCain said. "Because there's legitimate differences between myself and Senator Obama and Senator Clinton on what we should do in Iraq."
The Democratic presidential candidates want to set a date for withdrawal from Iraq, which McCain argues would lead to chaos and genocide in the Middle East.
"After we win the war in Iraq, and we are succeeding - and it's long and hard and tough, with enormous sacrifices - then I'm talking about a security arrangement that may or may not be the same kind of thing we had with Korea after the Korean war was over," he said.
He described how, at the time, the U.S. entered into a security agreement with South Korea, and the U.S. troop presence there served as a buffer and as a deterrent from further North Korean aggression.
"So it's too bad that they're deliberately falsifying, when there are legitimate differences," he said.
The other ad, run by the Democratic National Committee, says President Bush has talked about staying in Iraq for 50 years, then plays a clip of McCain saying, "Maybe 100. That'd be fine with me."
The announcer then says: "If all he offers is more of the same, is John McCain the right choice for America's future?"
At issue is McCain's answer, in January, to a question about Mr. Bush's theory that troops could be in Iraq for 50 years.
McCain said: "Maybe 100. As long as Americans are not being injured or harmed or wounded or killed, that'd be fine with me, and I hope it would be fine with you, if we maintain a presence in a very volatile part of the world where al Qaeda is training, recruiting, equipping and motivating people every single day."
Also Friday, McCain's campaign began running a health care ad in the presidential battleground state of Ohio. The campaign ran a similar spot this week in Iowa, another general election battleground.