Finally: Rudy Giuliani was caught red-handed.

After months of watching him wriggle out of tight spots on issues like abortion and gun control, opponents of the front-running former mayor say that his reversal on immigration policy has finally brought down the mayor’s impenetrable defenses and opened him up to attacks on everything from the consistency of his record, to his personal life, to the veracity of his remarks about Ground Zero.

They couldn’t be happier.

“It’s going to damage him pretty severely,” said Representative Tom Tancredo, a Republican presidential candidate running largely on an anti-immigration platform. “He was prepared to be hassled about the life issue and the gun issue, you knew that was going to happen, so he really had that down pat. But the immigration thing came out of the blue. Now he is dancing, trying to figure out what steps to take to evade this one too.”

“I think it is impossible for any presidential candidate to believe that there is Teflon,” said Kevin Madden, a spokesman for Mitt Romney. “Both the media and scrutiny by a national electorate will always guarantee that everything sticks at some point.”

Mr. Giuliani’s problems started when Mr. Romney broached the subject of immigration on Aug. 8 by calling New York, under Mr. Giuliani, a “poster child for sanctuary cities in the country.”

On Aug. 14, Mr. Giuliani, anxious to avoid appearing weak on an issue that had contributed greatly to the demise of John McCain’s campaign, boldly told voters in South Carolina that “I promise you, we can end illegal immigration.”

It was a bad calculation.

A day later, a rival campaign provided the liberal blog Talking Points Memo with a 1996 video showing Mr. Giuliani, complete with glossy comb-over, telling an audience at Harvard’s Kennedy School of government that “we’re never ever going to be able to totally control immigration to a country that is as large as ours.”

In the 1996 speech, which was quickly posted to YouTube and spread all over the Web, Mr. Giuliani goes on to say efforts to comprehensively stop immigration “might very well destroy the economy of the United States.”

Mr. Giuliani’s response was weak: He explained what looked to all the world like a blatant flip-flop by saying that advances in technology have created methods of fighting illegal immigration that were unimaginable when he made his 1996 remarks.

Since then, it’s been open season on Mr. Giuliani.

Mr. Tancredo said that for months he had been trying to “raise as much hell as I can and draw attention to this terrible, terrible hypocrisy,” but, he admitted, nobody was listening. All of a sudden, he said, something has changed. “You’ve got somebody with enough money to press it home: Romney.”

And Mr. Romney has spent money to do just that. On Aug. 21, the former Massachusetts governor released a new radio ad in New Hampshire and Iowa that says “immigration laws don’t work if they’re ignored. That’s the problem with cities like Newark, San Francisco and New York City that adopt sanctuary policies.”

“Essentially,” Mr. Madden explained, “what you have is a record that is abysmal.”

Mr. Romney has his own problems of credibility and inconsistency on abortion and gun control. For him, immigration was the ideal point of attack.

Now, that chink in Mr. Giuliani’s armor has created space for a wider variety of hits.

On Aug. 21, prospective Republican candidate Fred Thompson wrote a blog post called “A New York State of Mind” on his “I’m With Fred” Web site. In it, he scorned New York’s gun control efforts as a way “to force its ways on the rest of us,” wrote Thompson. “Now, the same activist federal judge from Brooklyn who provided Mayor Giuliani’s administration with the legal ruling it sought to sue gun makers, has done it again.”

Maria Comella, a spokeswoman for the Giuliani campaign, begs to differ with the rivals’ assessment.

“This is an indication of other candidates struggling to find relevancy where they don’t have any,” she said. “The bottom line is Mayor Giuliani is the front-runner in this race and the only candidate with executive experience and a track record to beat the Democrats in 2008.”

She added: “Mayor Giuliani is prepared to do battle when it comes to his record any day of the week. That is because it is a record of proven results.”

As for the Thompson blog post, the Giuliani campaign sent out a statement from communications director Katie Levinson saying, “Those who live in New York in the real world—not on TV—know that Rudy Giuliani’s record of making the city safe for families speaks for itself. No amount of political theater will change that.”

Robert Mosbacher, John McCain’s campaign chairman, declined to make any direct criticisms about Mr. Giuliani. But given Mr. McCain’s own brutal and campaign-endangering experience with the immigration issue, Mr. Mosbacher admitted “it helps McCain when attention like that is on someone else.”

Another prominent supporter of Mr. McCain put it more sharply, if somewhat hopefully.

“They start focusing on Rudy and he’s got only one place to go,” the supporter said. “And that’s down.”