DES MOINES, Iowa -- Rudy Giuliani invokes the 2001 terrorist attacks -- the tragedy that made him the face of U.S. resolve and put him on the national political map -- in a new ad campaign launched as the Republican tries to overcome a rough patch in his presidential candidacy.

"During the day of September 11, living through the things that I saw and observed. Immediately, when I saw people helping each other. I saw the picture of the firefighters putting the flag up at ground zero," the former New York mayor says in a TV commercial. "I said these are the children or grandchildren or great-grandchildren of the greatest generation. They have the same resolve. The same understanding."

Giuliani's emphasis on the 2001 attacks marks a strategy shift as he faces a series of likely defeats in Iowa, New Hampshire and other early contest states -- and as he seeks to stay relevant with his unconventional plan of favoring big states that come later in the primary season and offer more delegates.

Until recently, Giuliani held a commanding lead in national polls but his fortunes have soured and his standings in surveys have slipped as his rivals have risen.

He has campaigned away from the political epicenters of Iowa and New Hampshire, and, thus, hasn't been part of the daily campaign story line. He has faced a series of setbacks starting with his disgraced former police commissioner Bernard Kerik's federal indictment on multiple charges. And, he failed to change the dynamics of the race with a speech in Florida that his campaign billed as his closing argument.

At the same time, terrorism -- the issue with which Giuliani is perhaps forever linked for his steady posture as mayor on 9/11 -- gave way to pocketbook issues such as the economy in voters' minds.

Thus, Giuliani is trying to put his strongest issue back in the mix -- and remind voters why they have held him in such high regard -- as he seeks to regain momentum.

Like other candidates, he seized on the terrorism issue Thursday as he condemned the assassination of former Pakistan Prime Minister Benazir Bhutto, and said: "We have to redouble our efforts in that part of the world to make sure there's not a slip back to terrorists."

Campaigning in Florida, he defended the new ad, saying: "It's a part of my life that helps define me. It's not the only thing that defines me."

It starts Friday and is scheduled to run nationally on cable's Fox News Channel. It also will air in New Hampshire, and, for the first time, Giuliani will expand his advertising to Florida. He has made that state a priority but his monthslong double-digit lead in that state's polls has narrowed as rival Mike Huckabee gained ground.

In the commercial, Giuliani recalls the World War II era and shows pictures of that period, as well as 2001 images of firefighters, other first responders, and an American flag amid World Trade Center wreckage. As classical music plays, Giuliani strikes an optimistic tone and recalls U.S. achievements like the space program to illustrate the country's will in overcoming challenges.

Putting terrorism front-and-center, he says: "When you come and try and take away from us our freedom, when you try and come here and kill our people, we're one and we're going to stand up to you and we're going to prevail."

As he has campaigned, Giuliani has sought to strike a careful balance to avoid the appearance that he's exploiting the attacks for his own political gain.

Even so, he has drawn criticism. Some firefighter unions and family members of those killed are campaigning against him, and Democratic candidate Joe Biden has poked fun at him for his references to the attacks. Biden said the only words Giuliani's sentences amount to are "a noun, and a verb and 9/11."

Giuliani isn't the first presidential candidate to use the attacks in ads. Hillary Rodham Clinton was shown in one of her ads at the World Trade Center site. President Bush's re-election campaign was both criticized and praised by victims' family members for invoking the attacks in 2004.