BURBANK, Calif. — A former Massachusetts governor, Mitt Romney, is blaming Mayor Giuliani for aggravating the problem of illegal immigration by making New York into a magnet for illegal aliens when he was mayor.

“He was responsible for a sanctuary city. He did not oppose a sanctuary city. He welcomed illegal aliens to the city,” Mr. Romney told reporters during a campaign swing through southern California yesterday. “That sanctuary state of mind is one of the reasons we have so many illegal aliens in our country today. … I think he was incorrect in pursuing that course,” the former governor said. Messrs. Romney and Giuliani have been sparring over immigration issues in recent days as each man presses his campaign for the Republican presidential nomination.

“Once again, presidential trail Mitt Romney completely ignores the record of Governor Mitt Romney,” a spokesman for Mr. Giuliani, Maria Comella, replied last night. “Under Governor Mitt Romney, the number of illegal immigrants skyrocketed, while he recommended millions of dollars in state aid to numerous sanctuary cities and to companies employing illegal immigrants, not to mention the illegals working on his own lawn.”

Mr. Romney spoke to journalists following a town-hall-style meeting where he spent almost an hour taking questions from about 200 voters on subjects ranging from the gridlock in Washington to the health care plan he instituted in Massachusetts.

“What we did in Massachusetts proved to every governor and to those that watch health care that we can get everybody insured in this country without socialized medicine, without a government-run system, without new taxes required,” Mr. Romney said.

The presidential campaign of a former Republican senator, Fred Thompson of Tennessee, tried to turn that health plan into a liability for Mr. Romney by pointing out that yesterday was the first day on which those who fail to buy insurance become liable to pay a penalty to the state.

“At least I have a plan,” Mr. Romney shot back. “The last thing we need is politicians who can only criticize plans as opposed to describe their own solutions.”

Mr. Romney said he does not favor every aspect of the Massachusetts plan. When asked about the penalties, he seemed to say he supported them, but then pivoted to discuss an aspect of the plan the journalist had not inquired about. A real estate investor who attended the “Ask Mitt Anything” session, Corbin Russell of Los Angeles, asked Mr. Romney about how he would address the mortgage crisis that has stalled housing sales. “I just don’t think the president’s plan is good enough,” Mr. Russell said.

Mr. Romney asserted that lawmakers were partially responsible for the crisis by pressing lenders to write more loans in poor neighborhoods. “Congress was telling the bankers you better give out mortgages to people who are poor. …They pushed that harder and harder,” he said.

Ultimately, however, the former governor embraced Mr. Bush’s response to the crisis and offered no new specifics on how he would spur the housing sector. Mr. Russell, who is undecided and attended an event with Mr. Giuliani earlier this week, said later that he got the sense Mr. Romney would do more than Mr. Bush has. “I think he’d be more concerned,” the real estate investor said.

Most of Mr. Romney’s sniping at other Republicans came as he spoke with reporters. That left him free at the public event to display the sunny, amiable persona that contrasts with the dour demeanor often exhibited by his rivals.

Mr. Romney’s Mormonism came up on a couple of occasions yesterday. While the former governor does not dwell publicly on his faith, neither does he hide it. Moments after he began speaking yesterday, he pointed out an audience member who joined him on missionary work in Europe decades ago. “You probably know that I served my church for 2 years, 2 1/2 years in France,” Mr. Romney said.

Lest some view him as a zealot or killjoy because of his ties to the conservative religion, Mr. Romney shows flashes of irreverence. The former governor told the crowd that when he can’t get to sleep on the campaign trail, he sometimes opens the bedside drawer in his hotel room. “Thank goodness, the Gideons left me something that ultimately puts me to sleep,” he said, drawing a laugh while deftly underscoring that Mormons views themselves as part of mainstream Christianity.

Asked by The New York Sun if he sees bigotry in the unwillingness of some voters to support a Mormon, Mr. Romney demurred. “I respectfully acknowledge the rights of people to choose presidents any way they want to, but believe that my faith will be a source of strength for me in this race,” he said.