Updated

In the heated campaign for the Republican presidential nomination, saying one's opponent supports "sanctuary cities" can be a declaration of political war.

That's what Mitt Romney set out to do last week when he attacked frontrunner Rudy Giuliani for what Romney described as Giuliani's soft approach to illegal immigration when he was mayor of New York.

At question is Giuliani's decision to uphold a city ordinance that prohibited municipal workers from turning over information about illegal aliens to federal authorities, unless the immigrant in question was suspected of criminal activity. Being in the country is a civil infraction, according to federal law.

"Immigration laws don't work if they are ignored," Romney says in his latest radio advertisement. "That's the problem with cities like Newark, San Francisco and New York City that adopt sanctuary policies. Sanctuary cities become magnets that encourage illegal immigration and undermine secure borders."

Declarations by local officials that they will not inform the federal government of any illegal immigrants in the community have earned several towns the name "sanctuary cities." The term conveys a welcoming reassurance to illegal immigrants seeking jobs, housing or local government services.

Giuliani, who was New York's mayor from 1994 to 2002 has defended this type of "don't ask, don't tell" approach to illegals, saying that the decline in New York's crime crime rate when he was mayor would have been undercut if the estimated 400,000 illegal immigrants there had been forced to go underground or onto the streets for fear of being turned over to the feds or denied basic services.

Giuliani has advocated forceful new federal laws like uniform identification cards for all foreign visitors and students. He also has criticized the federal government for not controlling illegal immigration and enforcing immigration laws. Along the way, he has ardently denied that New York was ever a "sanctuary city."

"New York is the safest large city in America since Mayor Giuliani turned it around," Giuliani spokeswoman Katie Levinson said after Romney's latest attack. "It is not a haven for illegality of any kind."

Republican analysts say the topic is a tough one for candidates.

"I think Giuliani's position is suspect on this one," said John Stone, former deputy chief of staff for the late Rep. Charlie Norwood, R-Ga., and president of the Virginia-based Freedom Foundation, which advocates stricter border policies.

He said Giuliani and Romney have been forced to move further right in their effort to prove how tough they are on illegal immigration, and that Romney's latest attack could undercut Giuliani's support with the conservative base.

"How heartfelt is that change in position?" Stone asked. "I think voters will look at that."

When the Law is Not a Law

The federal government has full responsibility for enforcing immigration law and cannot compel states or localities to turn over illegal immigrants. On top of that, federal law mandates that local communities provide everyone, including illegal aliens, access to emergency medical care and a public education.

In practice, most state and local governments have some form of policy that guides them on what to do if an illegal immigrant commits a crime, including the point at which local law enforcement officials can demand a person's residency documents.

With the rapid uptick in the number of illegal immigrants entering the country over the past few years, many states and municipalities are now passing laws to give authorities more power to screen immigrants and deny some services. In some places, including parts of Northern Virginia, Arizona and Georgia, local officials are taking advantage of federal provisions to allow the local police to be "deputized," giving them broader power to arrest illegals.

On the opposite end of the spectrum, including in New York City, local governments are further limiting their role in enforcing federal immigration law. The difference in approach appears to be dividing the nation.

"It's a sleeper issue, especially with the Republican Party, because its roots are in the suburbs and rural America," said Craig Shirley, a veteran Republican strategist. He said "the issue has a lot of potency," because it cuts through typical demographic and cultural biases that split conservatives and liberals.

"These sanctuary cities smack of urban smugness, [saying] 'We know better than you, you unsophisticated hayseed,'" Shirley said.

No Sanctuary on the Road to the White House

Romney, who in most polls trails Giuliani and undeclared candidate Fred Thompson, is trying to use the issue of illegal immigration, and particularly sanctuary cities, to his advantage.

In his latest ad, an announcer claims that Romney "said no to driver’s licenses for those here illegally ... insisted on teaching our kids in English. And as president, Mitt Romney will cut back federal funds to cities that provide sanctuary to illegal immigrants."

"If someone is going to be running for the highest office in the land, then I would elect the person who respects the rule of law," said Aubrey Stokes, co-founder of Help Save Virginia, an umbrella group for several Washington-area organizations that oppose illegal immigration.

Stokes said Republicans will definitely weigh the presidential candidates' views on so-called sanctuary policies before casting a ballot. But, Stokes said, he is not certain how serious the charges leveled against Giuliani are,or whether Romney was any more effective against illegal immigration when he was governor of Massachusetts from 2002 through 2006.

"I would hope people would not pay so much attention to political posturing and pay attention to the facts themselves," said Stokes. "It's probably very important to set the record straight."

According to Boston newspapers and FactCheck.org, Romney was largely unsuccessful in trying to thwart his state's sanctuary status. For example, the cities of Cambridge, Orleans and Somerville were largely considered to be sanctuary cities during his tenure. "We find no evidence that Romney took a hard stance against those cities' policies as governor," said FactCheck.org.

On the other hand, Romney did veto a measure that would have afforded in-state college tuition to illegal immigrants and signed an agreement to give state police powers doled out by the Immigration and Customs Enforcement bureau of the Homeland Security Department. That didn't occur until six months before he left office, and it was "promptly rescinded by his successor," Democratic Gov. Deval Patrick.

Playing to the Right in '07, to the Middle in '08

Nonetheless, Romney appears confident that he has the upper hand in the debate, though it may not benefit him in the long run.

University of Washington professor Gary Segura, who specializes in Latino voter representation and elections, said the candidates' tussle over immigration — resulting in both candidates sounding ever more immovable on the issue — plays to the Republican base, but won't be so effective in the general election.

"Giuliani will try to dance around some of this stuff. The problem of course is, should he be nominated, he will absolutely change his tune by August of next year," Segura said. "Giuliani right now has to pander and that could be problematic for him."

Some conservatives, like Stokes, are pragmatic and recognize that as the frontrunner with crossover appeal and the best chance to beat Democratic leader Sen. Hillary Clinton, Giuliani's mayoral positions may be less important than the Republicans' opportunity to keep the White House.

"I like [Rep.] Tom Tancredo for his immigration stance," said Stokes, referring to the Colorado Republican whose presidential candidacy is based almost entirely on advocating tougher immigration laws. Stokes acknowledges that a "one-issue candidate" is typically a longshot.

"Some people are single issue voters, others aren't," said Danny Vargas, chairman of the Republican National Hispanic Assembly. "While immigration is an important issue, let's remember it's important among many issues that we have to grapple with."

Gary Lee, chairman of the Lee County Republicans in Florida, which is holding one of the earliest primaries on Jan. 29, said he was afraid the whole Romney-Giuliani exchange does very little for GOP prospects overall.

"It serves no useful purpose to eat your own, so to speak," he said. "We shouldn't throw stones."

The Associated Press contributed to this report.