The issue prompted the first big Romney-McCain skirmish, at the May 15 debate in South Carolina. Romney referred to an old immigration proposal that McCain supported with Sen. Edward M. Kennedy, D-Mass., not a name that engenders love among many Republican primary voters.

"My fear is that McCain-Kennedy would do to immigration what McCain-Feingold has done to campaign finance and money in politics," Romney said.

McCain has taken more and more to pointing out Romney's conversions, in an effort to portray him as a flip-flopper. At the debate McCain responded that he had not changed his position "because of the different offices that I may be running for."

As the new Senate accord was criticized by conservatives, Romney created an immigration television commercial last week, and McCain stepped up his response, calling the current immigration system "unacceptable."

Then he used the line about the small-varmint gun and the Guatemalans. That was a reference to Romney's recent attempts to portray himself as a lifelong hunter, despite his lack of hunting licenses, by saying that he hunted "small varmints" and to a report that Romney had used a lawn company that employed illegal immigrants from Guatemala.

Brian Jones, a spokesman for McCain, said, "Senator McCain is highlighting what he sees as an example of political opportunism."

"The race is mature enough now, and evolved enough now, where you can begin to draw clear distinctions between you and your opponents," Jones said

Add to that the personal factor. McCain and Giuliani have something of a political friendship. Giuliani, after all, bucked much of New York's Republican establishment in 2000 when it tried to keep McCain off the primary ballot. The two men have taken in a number of ballgames together, not to mention meals in Little Italy. And while Giuliani has adjusted his positions somewhat on key issues, with months to go before the first votes are cast, it is still early for them to begin squabbling.

The McCain-Romney relationship, though, seems downright frosty.

Nelson Warfield, a Republican consultant who worked as the press secretary for Sen. Bob Dole's presidential campaign in 1996, said he saw McCain's recent remarks as more evidence that he says what he wants, when he wants.

"I think with John McCain there is no three-second delay," Warfield said.

McCain's attacks are not without their own risks. To some extent, they elevate Romney, who still places third in many national polls. And they have already renewed questions about McCain's temper and temperament, something the Romney camp clearly relished.

Kevin Madden, a spokesman for Romney, said that while Romney was raising "substantive" questions about the immigration proposal, "other campaigns have spent the last couple of says lashing out in anger and taking shots."

Asked on the trail Tuesday about McCain varmint-gun salvo, Romney laughed it off, telling The Associated Press that "I have respect for Senator McCain, and I guess it just shows that even when he's wrong, he's amusing."