The war in Iraq sparked a war of words Wednesday between two presidential candidates -- Republican frontrunner John McCain and Democratic frontrunner Barack Obama.

The verbal battle also centers on a1-Qaeda the group blamed for the attacks on the United States on September 11th, 2001.

The stage was set at the Democratic debate in Cleveland, Ohio, Tuesday night.

Obama repeated his pledge to withdraw US combat troops from Iraq if he becomes president.

NBC's Tim Russert then asked Obama if Obama would reserve the right to go back into Iraq after withdrawing, if there were "any kind of insurrection (uprising) or civil war."

Obama responded, "As commander in chief, I will always reserve the right to make sure that we are looking out for American interests. And if al-Qaeda is forming a base in Iraq, then we will have to act in a way that secures the American homeland and our interests abroad."

In Texas Wednesday, McCain pounced on that last sentence.

"I have some news," he said. "Al-Qaeda is in Iraq."

McCain continued, "My friends, if we left, they wouldn't be establishing a base. They'd be taking a country. And I'm not going to allow that to happen."

In Ohio later in the day, Obama fired back.

"I have some news for John McCain," he said at a rally in Columbus. "There was no such thing as al-Qaeda in Iraq until (President) George Bush and John McCain decided to invade Iraq.

"They took their eye off the people who were responsible for 9/11," Obama continued, "and that would be al-Qaeda in Afghanistan, that is stronger now than at any time since 2001."

(Background: In the months leading up to the start of the Iraq war five years ago, President Bush linked the need for military action in Iraq to the September 11th attacks (also known as 9/11). He also implied that former Iraqi leader Saddam Hussein the dictator US troops removed from power was somehow linked to 9/11.

Since then, however, experts have established that there was no link between Saddam Hussein and the people responsible for the September 11th attacks. And experts have said that al-Qaeda did not have a presence in Iraq until after US troops invaded in 2003.

Obama and other critics of the war say the US invasion actually made the situation more dangerous, by allowing terrorists to get into Iraq and set up new outposts there.)

McCain responded on Thursday morning in Houston.

"Senator Obama said, 'Well, we shouldn't have gone in in the first place. And if we hadn't gone in in the first place, we wouldn't be facing this problem.'

"Well, that's history. That's the past," McCain continued. "What we should be talking about is what we're going to do now and what we're going to do now is continue this strategy, which is succeeding in Iraq."

But Obama wasn't finished. In Austin, Texas, Thursday, he said the war in Iraq is one of the reasons for the nation's current economic problems. And he once again linked McCain to the war.

"We are spending $12 billion a month," Obama said. "$12 billion a month. We cannot sustain that. You know, when John McCain says, you know, 'I'm willing to stay there (in Iraq) 100 years,' understand what that means. We are talking about trillions of dollars. Not billions trillions of dollars. And that is not sustainable."

For the record, McCain says Democrats are distorting a remark he made about being in Iraq for the next 100 years. He says he means there will probably be some sort of US presence in Iraq for a long time, not that the war itself will continue for 100 years.

Obama says McCain would be an extension of President Bush four more years of the current economic policies and four more years of the current strategy in Iraq.

This may be the beginning a long debate between these two men a debate that could last until the November election.