"Yes, I worry about it," he said Friday, responding to a question at a town hall-style forum. "And I know they pay attention, because of the intercepts we have of their communications."
The questioner asked if McCain feared al Qaeda in Iraq or another group might attack in an effort to aid the Democratic nominee, because Democratic Sens.and both favor a withdrawal of U.S. forces.
McCain told reporters later that al Qaeda remains smart and adaptable despite an increase of U.S. troops in Iraq.
"We have had great success with the surge, but to think they're not capable of orchestrating really strong attacks ... I think is an underestimation of the enemy," McCain said.
"We still have the most lethal explosive devices coming across the border from Iran into Iraq," he said. "We still have suicide bombers landing at the airport in Damascus and coming into Iraq as we speak.
"So I would not be surprised if they make an attempt. I believe that we can counter most of it, as we are countering. But there will still be spikes and difficulties and challenges associated with this conflict. Otherwise, I'd be advocating that they come home," he said.
McCain plans to make his eighth trip to Iraq this weekend on a weeklong overseas trip that includes Israel, Britain and France.
He was campaigning Friday in Pennsylvania, which holds presidential primary elections on April 22. Clinton and Obama have claimed the spotlight in the state; McCain sewed up the Republican nomination with victories March 4 in Ohio and Texas.