The leading contenders for the 2012 Republican presidential nomination appeared together onstage for the first time Monday. But they used the debate to train their fire on President Obama rather than to define their differences.

Given opportunities to critique one another’s stances, the seven competitors repeatedly deflected the questions to attacks on the president.

Former Minnesota governor Tim Pawlenty was asked, for instance, about his description on Sunday of the new national health-care law as “Obamneycare” — an effort to link Obama’s overhaul to the one that Mitt Romney championed and signed into law in 2006 when he was governor of Massachusetts.

Pawlenty demurred, and refrained from any direct criticism of the Romney plan while the two stood on the same stage. He said he was merely quoting Obama, who had previously said that he had modeled the federal law on the one in Massachusetts.

“The president is going to eat those words,” Romney retorted.

Romney called the new law “a huge power grab by the federal government,” and repeated his vow to repeal it if elected.

The biggest surprise of the evening was the announcement by Rep. Michele Bachmann (R-Minn.) that she has filed paperwork to begin her campaign. She had previously indicated that she would not make a formal announcement until later this month.

The debate — held at Saint Anselm College and sponsored by CNN, the New Hampshire Union Leader and WMUR-TV — was officially the second of the primary campaign season. But only five candidates showed up last month in South Carolina, and with the exception of Pawlenty, they were all long shots.

Monday’s forum included nearly all the leading contenders. But most of them remain largely unknown nationally, a factor that could explain their reluctance to go at one another. At this early stage, they are introducing themselves to a nationwide audience, and testing their competitors’ strengths and vulnerabilities.

It is a Republican field unlike any other in generations. None of the candidates has been able to establish himself or herself as an overwhelming favorite. In the normal order of things, Romney would hold that position, by virtue of his organization, his fundraising network and the exposure he received from his 2008 run for the nomination.

But although Romney holds a lead in most national surveys, it has been a narrow one, particularly in comparison with the double-digit advantage that the front-running GOP contender normally has at this point.

In part, that reflects the fact that many conservatives are uneasy with parts of his record in Massachusetts, where his chief accomplishment was the passage of the health-care plan that became the model for Obama’s overhaul.

The Republican field also has been unsettled by the insurgent energy of the tea party movement, which represents a rebellion against the established order of GOP politics.

Romney was seeking to keep his focus on his strongest issues: the economy and jobs. New Hampshire, where he suffered a crippling defeat in 2008 to Sen. John McCain (Ariz.), is crucial to his hopes in 2012.

“New Hampshire has proven the issue people care most about is getting the economy going again,” Romney said near the conclusion of the debate.

Monday’s forum was Romney’s biggest opportunity yet to demonstrate that his background in business, where he specialized in turning around struggling companies, gives him a command on the economy that is unmatched by the other candidates, most of whom have spent the bulk of their careers in government.

“There is a perception in this country that the government knows better than the private sector, that Washington and President Obama have a better view of how an industry ought to be run,” Romney said, in explaining his opposition to 2009 auto industry bailouts. “Well, they’re wrong.”

Pawlenty has been struggling to make himself the leading alternative to Romney. Although he has been campaigning quietly and energetically in the early-voting states, the low-key former Minnesota governor has not made much of an impression nationally.

He defended the economic plan that he announced last week against criticism that its promise of 5 percent annual economic growth is too optimistic.

“This president is a declinist,” Pawlenty said. “He views America as one of equals around the world. We’re not the same as Portugal, we’re not the same as Argentina. This idea that we can’t have 5 percent growth in America is hogwash. It’s a defeatist attitude. If China can have 5 percent growth and Brazil can have 5 percent growth, then the United States of America can have 5 percent growth.”

The newer energy in the party was represented onstage as well, particularly by Bachmann, a tea party favorite and the lone woman among the seven.

She noted that she was the first House member to introduce legislation to repeal the health-care law and the Obama administration’s financial regulatory overhaul.

“I fought behind closed doors against my own party” on the $700 billion Wall Street bailout in 2008, Bachmann added, describing the George W. Bush administration’s initiative as “a wrong vote then. It’s continued to be a wrong vote since then. Sometimes, that’s what you have to do. You have to take principle over your party.”

Bachmann has attracted considerable grass-roots support during her recent visits to Iowa, and she is likely to make the first-in-the-nation caucus state the centerpiece of her strategy to win the Republican nomination. She also claims something of a home-court advantage, having been born in Waterloo.

To coincide with her announcement, Bachmann released a video calling on supporters to volunteer, donate and tell their friends to join a “winning team.”

“This is the first day of taking our country back,” she said in the one-minute clip. “I’ve worked very hard to bring your voice to the halls of Congress. Now, I want to take your voice into the White House, where it hasn’t been heard for a very long time.”

Another candidate who had hopes of appealing to the tea party sentiment was former House speaker Newt Gingrich (Ga.), but his challenge in his first debate was more fundamental than that. After his campaign collapsed last week with the abrupt exodus of virtually its entire leadership, Gingrich was attempting to make a convincing case that his candidacy is still alive.

It did not appear that his campaign’s setbacks have dampened Gingrich’s characteristic zeal. “The Obama administration is an anti-jobs, anti-business, anti-American-energy destructive force,” he said.

The remaining men on stage — former senator Rick Santorum (Pa.), Rep. Ron Paul (Tex.) and former Godfather’s Pizza executive Herman Cain — are considered long shots at best. But although they had little to lose by taking jabs at the top-tier contenders, and potentially had much to gain from national exposure, they also passed up opportunities to attack the front-runners.

Cain was asked about polls showing that Republicans view the GOP field as weak, and dismissed them as indicating that voters have not yet gotten to know the contenders. “This really is a good field of candidates, in my opinion,” he said.

Santorum was asked about Romney’s shift from pro-abortion rights to anti-abortion. He instead chose to stress his own staunch opposition to legal abortion, and said the candidates’ authenticity on the issue is “a factor that should be determined.”

Romney said: “I am firmly pro-life.”

Missing from the stage was at least one likely contender, former Utah governor Jon Huntsman Jr., who spent the weekend in the state doing what was by all appearances campaigning. But he declined to participate, noting that he has only recently returned from his post as Obama’s ambassador to China and is still mulling over whether to run.

The field could continue to reshape itself. One of those considering a run is Texas Gov. Rick Perry, whose top strategists were among those who left the Gingrich campaign last week.

Former Alaska governor Sarah Palin also has not yet announced her plans. Pawlenty, who was passed up to be McCain’s running mate in favor of Palin, was asked whether she had been a superior choice to Obama’s pick of Vice President Biden.

He described Palin as “a remarkable leader . . . qualified to be president . . . and equally or more qualified than Joe Biden.”