The nature of the ads has something to do with it. Besides the ill-timed Iran ad, Giuliani ran a spot touting his (truly extraordinary) work as New York City mayor. But Giuliani left the mayor's office six years ago, and was first elected -- and began the city's turnaround -- 14 years ago. Voters don't just want to know what you've done lately, but what you'll do for them in the future.
Over the weekend, Giuliani went to Florida to try to relaunch his campaign with a speech focused on his forward-looking "12 commitments" as president. He didn't mention the one about reducing abortions. When Giuliani was high in the polls, pundits speculated that the war on terror was the new social issue, more important to Republican voters than abortion or gay marriage. Huckabee's rise shows that social conservatives are still animated by their traditional issues, and Giuliani has little to say to them.
It's possible that the Republican field stays fragmented enough that Giuliani can win Florida on Jan. 29 and the big states where he is still strong on Feb. 5. But his scenario depends on a fractured party that he will have trouble putting back together again given his fundamental disagreements with a large part of the GOP base. In Florida, Giuliani said: "I don't just pray for miracles. I don't just hope for miracles. I expect miracles." He might have to.