Romney Parries Rivals With Negative Ads When Gov. Rick Perry began rising in the polls, Mitt Romney was ready with a potent debate-night assault: under Mr. Perry, illegal immigrants in Texas received a $100,000 tuition break. Mitt Romney, in North Dakota on Thursday, has effectively wielded negative attacks against political opponents over the years. As Newt Gingrich surged, the Romney campaign dispatched colleagues of Mr. Gingrich, a former House speaker, to ridicule him as "narcissistic" and "erratic" in conference calls that required a sardonic pass code: "Newt Gingrich is an unreliable leader." And with Rick Santorum persisting as a threat as 10 states prepare to vote on Tuesday, Mr. Romney unleashed an ad in Ohio on Thursday mocking him as an unprincipled flip-flopper. The Romney campaign's shortcomings have been on vivid display in recent weeks, from verbal stumbles to a failure to stir the passions of the Republican base. But even his battered rivals acknowledge that Mr. Romney is proving unusually adept at defining, diminishing and disqualifying a serial cast of challengers through relentless attacks. His campaign has deployed every tactic in the negative-campaign playbook. It has issued Twitter messages poking fun at Mr. Gingrich's penchant for rhetorical excess (with the hashtag #grandiosenewt). It created digital slogans and a letterhead disparaging Mr. Santorum's long career in government ("Rick Went to Washington," they read, "and he never came back"). It created dozens of Web videos denigrating President Obama's economic leadership ("Obama isn't working"). And it benefited from the advertising onslaught unleashed against Mr. Romney's rivals by a "super PAC" backing him. As successful as the strategy has been, though, it has raised questions about Mr. Romney's role in turning the primary process into something akin to a civil war, even as it has demonstrated a ferocious, whatever-it-takes style that could hearten Republicans if Mr. Romney ends up in a general election matchup against Mr. Obama. "It's clear the negative ads are what's keeping this guy alive," said Nelson Warfield, a Republican strategist who worked for Mr. Perry. It seems like Republican primary voters will not vote for Mitt Romney unless they are forced into it. And the way they're forced into it is when he beats the other guy senseless. Throughout the campaign, Mr. Romney has also been on the receiving end of attacks from his Republican rivals as well as Democrats. But his aggressive style has been apparent since his first days in politics. For all that he can appear stiff and scripted at times, Mr. Romney has never shied away from deploying pointed and colorful broadsides against his opponents. In his 1994 campaign against Senator Edward M. Kennedy in Massachusetts, it was Mr. Romney, a private equity executive new to politics, who fired off the first negative commercials. One of them featured grainy, slow-motion, black and white images of Mr. Kennedy painfully easing his body onto a park bench, before cutting to a full-color video of Mr. Romney. The message was not lost on viewers: Mr. Kennedy, then 62, was old and out of shape, a fixture of the past; Mr. Romney, 47, was young and vigorous, a promise of the future. Angry aides to Mr. Kennedy called it "video sleaze." Robert M. Shrum, Mr. Kennedy's admaker, recalled thinking: "You could see people sitting around in a room and laughing as they put it together. It was smarmy. But the commercials - amplified by ads challenging Mr. Kennedy's Congressional record on crime and job creation - helped turn what many analysts had expected to be a blowout into a surprisingly competitive race. "We took on water because of it," Mr. Shrum said. They were effective. In 2002, when he ran for governor of Massachusetts, Mr. Romney vastly outspent his Democratic opponent, State Treasurer Shannon O'Brien, whom he depicted in debates, speeches and ads as a self-dealing and entrenched vestige of a bankrupt Boston political culture.