You know things have gotten serious in a campaign when the candidates are insulting the sizes of one another’s guns. And you know, in a crowded field such as the one for the Republican Party’s nomination for president, the candidate who’s winning is the one who isn’t playing the game.

Take, for instance, the verbal assault unleashed by Senator McCain of Arizona yesterday against the former governor of Massachusetts, Mitt Romney. Asked by this reporter during a conference call what he made of attacks on the immigration compromise he engineered last week, Mr. McCain didn’t hold back.

“In the case of Governor Romney, you know, maybe I should wait a couple of weeks and see if it changes, because it’s changed in less than a year from his position before,” the senator said of Mr. Romney’s position on immigration. “And maybe his solution will be to get out his small-varmint gun and drive those Guatemalans off his lawn.”

It was a one-two-three punch. A hat trick. A turkey. First, a right hook at Mr. Romney’s various flip-flops on social issues (abortion, gay rights, etc.). Second, a slap shot at Mr. Romney’s now famous exaggeration of his status as a “lifelong hunter” (which he had to clarify: “I’ve always been a rodent and rabbit hunter, small varmints, if you will.”). Third, a perfect strike on Mr. Romney’s having once hired a landscaping company that employed illegal immigrants.

But, while it may have been a fun line, was it good move for Mr. McCain?

Things have been getting testy between the respective second-place and fourth-place candidates (the undeclared Fred Thompson is in third place) nationally for the Republican nomination. At last Tuesday night’s debate down in South Carolina, Mr. Romney said the McCain-Kennedy bill would do for immigration what the McCain-Feingold bill had done for campaign finance (I’m not sure what this metaphor means, but conservatives typically hate both bills). Mr. McCain countered: “I haven’t changed my position on even-numbered years, or changed because of the different offices that I may be running for.” The studio audience greeted that riposte with a sitcom-style “Ooooooo …”

As Messrs. McCain and Romney attempt to gouge each other’s eyes out, however, the front-runner, Mayor Giuliani, is able to float above it all. Or at least climb over his competitors’ battered corpses.

“Rudy has made clear there is no place for personal attacks in this race,” Mr. Giuliani’s communications director, Katie Levinson, told me yesterday. “He considers both McCain and Romney friends and was happy to campaign for both of them when they asked him to.”

Further, she added: “We believe this election is about staying on offense in the terrorists’ war on us and keeping our economy strong. There is a clear choice between the Republican candidates and the Democratic candidates on both these issues and we will continue to point them out to voters.”

Indeed, as Messrs. McCain and Romney have kept their respective small-varmint guns trained on each other, Mr. Giuliani has had bigger game in his sights: Senator Clinton. Both in the debate in South Carolina and at a Republican Party dinner in New York City on Thursday night, Mr. Giuliani laid into Mrs. Clinton for a comment she made back in 2004 (with regard to repealing the Bush tax cuts): “We’re going to take things away from you on behalf of the common good.” How crazy is it, he asked the New York Republican audience, that the Democrats believe this stuff? The audience ate it up.

At that same New York Republican Party dinner, Mr. Giuliani laid it on thick for Mr. McCain. “I admire Senator McCain greatly, he is a very, very good friend,” Mr. Giuliani said. “If it weren’t for another candidate, I might actually be supporting him for president of the United States.”

Killed by kindness, Mr. McCain had to respond in kind later in the evening. “I was proud of Giuliani’s comments,” he said of the former mayor’s exchange with Texas congressman Ron Paul at the last debate, in which Mr. Giuliani took umbrage at the implication that American foreign policy was to blame for the attacks of September 11. “I was proud of him as an American.”

To be fair, Mr. Giuliani can afford to be above it all because he is above them all in the polls. But the typical path out of the political gutter is to take shots at those above you, not those below you — which only makes it all the more puzzling why Mr. McCain is firing off wildly at such small game.

rsager@nysun.com