Similarly, he said that, while he supported public financing of abortion for poor women in New York, in other states, “people can come to a different decision.”

Mr. Romney sought to explain his shift on the issue from being in favor of abortion rights, when he was running for office in Massachusetts, to running as a strong opponent now.

“I’ve always been personally pro-life, but for me there was a great question about whether or not government should intrude in that decision,” Mr. Romney said. It was the debate over cloning in his state that pressed him to change his mind, he said.

“I said, ‘Look, we have gone too far; it’s a brave new world mentality that Roe v. Wade has given us,’ and I changed my mind. I took the same course that Ronald Reagan and George Herbert Walker Bush and Henry Hyde took. And I said I was wrong and changed my mind and said I’m pro-life. And I’m proud of that and I won’t apologize to anybody for becoming pro-life.”

The 90-minute debate was sponsored by MSNBC and Politico.com, employing a mixture of questions from two moderators — Chris Matthews of MSNBC and John Harris of Politico.Com — with questions over the Internet from viewers.

On an issue of obviously great interest, the candidates raced to endorse various forms of tax cuts. Mr. McCain called for the repeal of the alternative minimum tax, saying it was hurting middle-class families, while Mr. Giuliani called for adjusting that tax — presumably by exempting more middle-class families — while repealing the estate tax. Mr. Romney called for repealing the capital gains tax on middle-class families.Mr. Tancredo, who has built his campaign around a strong call for tough measures against illegal immigration, criticized his opponents — he did not mention any by name, though his remarks appeared directed at Mr. Giuliani and Mr. McCain — for what he said was equivocation on the issue.