The contrast could hardly be more striking. Sen. John McCain and former Massachusetts Gov. Mitt Romney have been mixing it up on the trail with increasing intensity of late, from their feisty exchange at the last Republican debate to their arguments over immigration, which prompted McCain to suggest acidly this week that Romney's solution might be "to get out his small-varmint gun and drive those Guatemalans off his lawn."

Standing on the sidelines, safely out of the line of small-varmint gunfire, has been Rudolph W. Giuliani, the Republican who is still leading in most national polls.

If McCain and Romney have been skirmishing, McCain and Giuliani have something between a nonaggression pact and a mutual admiration society going. When they both appeared at a fund-raiser in New York last week, McCain praised Giuliani's debate performance, and Giuliani went so far as to say of McCain that "if it weren't for another candidate, I might actually be supporting him."

It is an unusual dynamic, to have the candidates placing second and third in most national opinion polls engaging one another fiercely, while allowing the front-runner a wide berth. But campaign officials and outside political consultants said that McCain's willingness to directly engage Romney appeared to stem from a combination of tactical, political, and, to a lesser extent, personal considerations.

Tactically, McCain and Romney are in direct competition because they have both invested enormous resources to compete in the early nominating contests in Iowa, New Hampshire and South Carolina. Giuliani, though, seems to be leaning toward competing more heavily in the large states, including California and New York, holding their primaries on Feb. 5. Several state-by-state polls have shown Romney gaining ground — even holding leads — in Iowa and New Hampshire.

That, some consultants said, could make McCain more likely to criticize Romney by name in an effort to stem his progress.

"I think it's a New Hampshire strategy, more than anything else," said Edward J. Rollins, a Republican consultant.

On top of that, Romney and McCain are competing for conservative support. To some extent, that base of voters is up for grabs this year because of Giuliani's current support of abortion rights, Romney's former support of abortion rights and the distrust with which many conservative groups view McCain because of his sponsorship of campaign finance legislation and his initial opposition to President Bush's tax cuts.

In recent days McCain's role in drafting the immigration proposal has galvanized many conservatives who fiercely oppose it, creating an opening for Romney.