Sensitive to criticisms like one Senator Joseph R. Biden Jr., Democrat of Delaware, leveled  that for Mr. Giuliani a sentence was “a noun and a verb and 9/11”  the campaign worked to show other dimensions. Over the summer and fall he made policy proposals, talked about his record as mayor of New York City, and reached further into his biography to highlight his career in the Justice Department in the 1970s and 1980s. For months, he was more likely to talk about supply-siders than suicide bombers.

Now, though, as Mr. Giuliani’s standing in the polls has slipped and he risks being sidelined by his decision not to compete fully in the Iowa caucuses, his campaign is returning to his signature issue. The shift began this month when he retooled his stump speech in Florida, with more references to Sept. 11 than usual. A campaign video shown on the Internet and at rallies includes an image of two terrorists in head scarves with guns. And he is running a minute-long advertisement focusing on Sept. 11.

“It is part of my life,” Mr. Giuliani said this week about the advertisement. “It is part of my life that helps to define me. It isn’t the only part of my life. But it would seem to me that maybe the critics want you to, like, remove a part of your life in which people have every right to draw judgments about how you would handle a crisis, how you would handle a difficult situation, how you would handle terrorism.”

The Bhutto assassination offered Mr. Giuliani a chance to appear on several news programs to talk about something other than the uncomfortable subjects that have dogged him recently, including the indictment of his former police commissioner in New York, Bernard B. Kerik, and the health scare that landed him in a St. Louis hospital last week.

But the renewed focus on Sept. 11 comes as Mr. Giuliani’s Republican opponents are more willing to challenge his argument that his experience after Sept. 11 gives him special insight on terrorism.