Fred Thompson sharpened the mood of the Republican presidential debate when he unveiled his anti-immigration policy in Collier County, Fla., yesterday, flashing a rare glint of steel at his closest rivals, Mayor Giuliani, Mitt Romney, and Senator McCain.

The former Tennessee senator’s proposed measures to curtail illegal immigration exploited a perceived weakness in his main opponents by drawing attention to their more generous approach to an issue that excites the Republican base like no other.

In an attempt to wrest the issue of national security from the front-runner, Mr. Giuliani, Mr. Thompson said defending the borders and preventing terrorism were linked.

“A small amount of nuclear material could do a lot of damage in the wrong hands,” he said. “It makes you wonder why a terrorist would bother going through an airport or a port … when we have an open border.” His campaign issued figures suggesting that 22% of crime is committed by illegal immigrants.

“You’ve clearly been swamped with a particular kind of problem because the federal government, in large part, has let you down and has not done their part,” he said.

Mr. Thompson took a clear swipe at Mr. Giuliani by threatening to cut off all federal grants to “sanctuary cities” and states, like New York, that fail to report illegal immigrants or offer them public benefits.

“Some of our cities in this country, for their own individual reasons and notions, have basically said to their locals, ‘You can’t cooperate with federal authorities. If you run across illegal aliens, you cannot cooperate with” the federal government, “you cannot reveal them to federal authorities,'” Mr. Thompson said. “That’s wrong. I propose that we cut off some discretionary funding to those cities.”

Other measures Mr. Thompson advocated were the rejection of amnesty for the approximately 12 million illegal immigrants already in America; more severe enforcement methods through existing powers to find, prosecute, and expel them; the bolstering of border security; a national identity card scheme; and more strict entry and exit regulations.

“There’s not a lot of new legislation that needs to be passed,” Mr. Thompson said. “We need to enforce the laws that are on the books. There are laws against illegal immigration, there are laws to secure the border, there are laws against sanctuary cities, there are laws against publicly funding illegals, and that law is being disregarded.”

“Without illegal employment opportunities available, fewer illegal aliens will attempt to enter the country, and many of those illegally in the country now likely will return home,” he said. “Self-deportation can also be maximized by stepping up the enforcement levels of other existing immigration laws.

“This course of action offers a reasonable alternative to the false choices currently proposed to deal with the 12 million or more aliens already in the U.S. illegally: Either arrest and deport them all, or give them all amnesty. Attrition through enforcement is a more reasonable and achievable solution, but this approach requires additional resources for enforcement and border security.”

But it was Mr. Thompson’s comments about “sanctuary cities” that provoked the loudest rejoinder from Mr. Giuliani, who has been grappling with Mr. Romney, a former governor of Massachusetts, on the issue. Mr. Thompson directed his principal fire at the former New York mayor’s legal challenge to a Senate measure in 1996 that attempted to outlaw “sanctuary cities.”

“In 1996, we passed a bill while I was in the Senate that outlawed sanctuary cities,” Mr. Thompson said. “Mayor Giuliani went to court to defeat that law, and unfortunately he won. But there are still, across the country now, many sanctuary cities which are in violation of the law. I just don’t think they ought to be able to do that with federal money.”

The Giuliani campaign’s communications director, Katie Levinson, responded in a statement by asking: “Where was Fred Thompson when he had the chance to tackle illegal immigration and fix a broken system? He was voting against $1 billion to combat illegal immigration at the borders, against stricter employment verification, and for giving illegal immigrants more benefits than we give legal immigrants. That’s not consistent or conservative.”

The Thompson campaign offered an instant riposte, pointing out that just two Republicans in the Senate had backed the 1996 measure and that their candidate considered the proposed law a back-door means of introducing a national ID system.

Mr. Giuliani “is basically criticizing us for being Republican, once again aligning himself with Senate Democrats,” a Thompson campaign spokesman, Jeff Sadosky, told NBC.

“Most Republicans, actually, were a little concerned about a national ID system,” Mr. Thompson said. “The technology was not there. We’ve come a long way since then, and of course 9/11 has happened since then. So I think it’s time that we had a system, like the one they call the eVerify system, where employers can very quickly determine whether or not somebody applying for a job is legal.”

The Romney campaign dismissed Mr. Thompson as a latecomer to the sanctuary cities debate. “Governor Romney has been the strongest candidate when it comes to demanding that our existing immigration laws are enforced,” a spokesman, Kevin Madden, said.