Rudy Giuliani with Mitt Romney outside a NYC firehouse

Rudy Giuliani returned to the national stage today in a familiar role: America's Mayor.

He won that nickname for the reassuring leadership he provided to New York and to the nation in the chaotic hours after the Sept. 11 attacks.

He put that image to use again today, this time standing with likely Republican presidential nominee Mitt Romney on the first anniversary of the U.S. military raid that killed Osama bin Laden.

But Giuliani's appearance - at a New York firehouse that lost 11 men in the collapse of the World Trade Center on Sept. 11 - carried special significance this time.

It came as Romney has been on the defensive, trying to beat back suggestions by President Obama and Vice President Biden that Romney would not have ordered the raid on bin Laden's Pakistani compound had he been president.

Giuliani vouched for Romney today, declaring, "Mitt Romney, anyone else, would have made the same decision." Giuliani also said that Obama deserves credit for ordering that raid, "but I wish he wouldn't use it as a source of negative campaigning. I think that's a big mistake."

Giuliani was wrapping up his second term as mayor when two planes crashed into the Trade Center's towers. He has been a potent symbol and weapon for the Republican Party ever since, even though his own candidacy for president fizzled four years ago.

He has endorsed Republicans around the country and gave a rousing speech for then-President George W. Bush at the 2004 Republican convention - held, not-so-coincidentally in New York City.

"Without really thinking, based on just emotion, spontaneously, I grabbed the arm of then-Police Commissioner Bernard Kerik and I said to him, 'Bernie, Thank God George Bush is our president,' and I say it again tonight, 'Thank God George Bush is our president,'" Giuliani told the convention crowd.

Political analysts said Giuliani's appearance with Romney couldn't hurt the Republican nominee, but they questioned how much it would help. Some said that by ordering the successful raid on bin Laden, Obama's anti-terrorism credentials now trump Giuliani's.

"The bottom line is: Obama killed Osama. In essence, Rudy is now the victim. He has been eclipsed as the face of defiance to al Qaeda and Osama. Obama was the one who killed the trigger," said Douglas Muzzio, a professor at the Baruch College School of Public Affairs.

But the event did not go exactly as planned, thanks to a New York phenomenon the former mayor knows all too well. A heckler repeatedly screamed "Mitt Romney is a racist!" as the former Massachusetts governor briefly spoke and took questions.  Romney never acknowledged the woman, who could be heard on television broadcasts of the event.

Giuliani's appearance with Romney was his first since the former mayor endorsed Romney last week. The two have had a rocky relationship. They were rivals during the 2008 presidential race, and late last year Giuliani ridiculed Romney as a serial flip-flopper who would be eviscerated by Obama if he became the Republican nominee.

"What Barack Obama will do with that [is say] 'this is a man without a core, a man without substance, a man that will say anything to become president of the United States,' Giuliani said then.

All that was forgotten today.

"I'm really glad that Gov. Romney selected this particular firehouse as a place to pay tribute to our firefighters, who after all were the first responders to this terrorist war against us, which is a war that still continues and that we still have to be vigilant about. Gov. Romney certainly understands that and has been from the very beginning a leader in the effort to make certain that America remains safe," Giuliani said.