Republican presidential candidate Mitt Romney aired a new television ad in Iowa casting himself as tough on terrorism, confronting one of the central themes of rival Rudy Giuliani's campaign.

In the 30-second ad, the former Massachusetts governor vows to strengthen U.S. intelligence agencies, increase the size of the military by at least 100,000 troops and to monitor calls made to the United States by suspected Al Qaeda associates.

The ad, which aides said had been prepared some time ago and will run statewide, is airing after Giuliani pounced on Romney's debate answer this week on whether to seek congressional authority before initiating action against Iran.

The ad also comes as Democratic lawmakers are moving a government eavesdropping bill through the House of Representatives. The Bush administration objects to the legislation because it does not give retroactive immunity to U.S. telecommunications companies that assisted the government conduct surveillance without court orders.

"It's this century's nightmare, Jihadism -- violent, radical Islamic fundamentalism," Romney says to the camera from the yard outside his Belmont, Mass., home.

"Their goal is to unite the world under a single Jihadist caliphate. To do that, they must collapse freedom-loving nations like us."

Romney is expected to discuss similar themes of national security and terrorism when he speaks before the Republican Jewish Coalition on Tuesday in Washington.

Giuliani, who has capitalized on his role as New York mayor during the 2001 terrorist attacks, and presidential candidate John McCain have both criticized Romney for suggesting that lawyers would have a role in determining whether Congress needed to approve a military strike against Iran's nuclear sites.

During a debate this week, Romney said he would talk to attorneys about Congress' role, but added: "Obviously the president of the United States has to do what's in the best interest of the United States to protect us against a potential threat."

In a letter to the editor of the Wall Street Journal on Friday, Romney defended his position, saying a president must act to protect Americans. "As to what involvement Congress may have, that's a matter for legal consideration," he wrote.

"When time and circumstances permit," he added, "I would indeed seek the involvement of Congress as required by law and the Constitution."