CONCORD, N.H. — Republican Mitt Romney assailed his two leading presidential rivals yesterday, criticizing Senator McCain’s stance on immigration and dismissing Mayor Giuliani’s support for gun control, abortion, and gay rights as a losing combination in the GOP primary.

Running a distant third in national polls, Mr. Romney used a campaign stop in New Hampshire to challenge Mr. McCain’s support for stronger border security paired with an eventual path to citizenship for some of the 11 million illegal immigrants in America.

Also, in an interview with the Christian Broadcasting Network, the former Massachusetts governor criticized Mr. Giuliani for his moderate stands on abortion, gays, and gun control — even though Mr. Romney expressed similar views in previous campaigns.

Mr. Romney began a series of appearances in New Hampshire, the first presidential primary state, by targeting Mr. McCain on immigration.

“I don’t agree with it; I think it’s the wrong course,” Mr. Romney said at the outset of four appearances in the first presidential primary state. “I do not believe amnesty is the right course for the 11 or 12 million illegal immigrants who are living here. It didn’t work in the 1980s; it’s not going to work in the 2000s either.”

Mr. McCain, who announced Wednesday night that he would formally become a candidate next month, had pushed legislation with Senator Kennedy, a Democrat of Massachusetts, that categorized illegal immigrants and established different criteria for them to remain in America legally.

Mr. Romney said he opposes amnesty, favors securing the American-Mexican border with a fence, and wants to institute an employment verification system through high-tech identification cards.

“If we can do that, then we can solve our problem with immigration. The answer is not amnesty,” Mr. Romney said to hearty applause from an audience of more than 150 gathered at New Hampshire Technical College.

In Utah for a fund-raiser, Mr. McCain responded to Mr. Romney’s criticisms.

“I think we need a comprehensive approach to it [immigration]. So does the president of the United States.

None of us support amnesty and we need to secure our borders,” the senator said.

McCain spokesman Matt David cited the discrepancy in Mr. Romney’s statements on the issue, citing his comment last March in which the former governor said he doesn’t believe in rounding up 11 million people and forcing them at gunpoint from the country.

In December, Mr. Romney was embarrassed when it turned out he had used a lawn care company that hired illegal workers.

In the interview posted on the CBN Web site, Mr. Romney said of Mr. Giuliani: “He is pro-choice, he is pro-gay marriage, and anti-gun. That’s a tough combination in a Republican primary.”

He also criticized in the interview Mr. McCain for opposing a constitutional amendment to ban same-sex marriage.

In Utah, Mr. McCain responded, saying: “I believe in the sanctity of marriage between a man and a woman, but I believe the states should decide.”

In his criticisms, Mr. Romney drew renewed scrutiny to his own past positions.

In previous campaigns, Mr. Romney has said that regardless of his own personal beliefs, abortion should be safe and legal. He changed his position about two years ago and now describes himself as “pro-life.”

In 1994, he wrote a letter promising a gay Republican group he would be a stronger advocate for gays and their rights than his liberal opponent, Mr. Kennedy, although Mr. Romney says he has always opposed gay marriage.

In the past, Mr. Romney emphasized his support of gun-control measures. In 1994, he said: “I don’t line up with the NRA.” In August, he became a card-carrying National Rifle Association member.

Responding to the criticism, Giuliani spokeswoman Katie Levinson said: “Mayor Giuliani has the utmost respect for Governor Romney.

The mayor’s position on gay marriage has been both clear and consistent — he does not support gay marriage and believes marriage should be between a man a woman.”