Rarely has a candidate who is seen as a front-runner for his party’s presidential nomination been as invisible as Mitt Romney.

That alone made it noteworthy when the former Massachusetts governor showed up here Friday night to participate in a forum with four other prospective candidates — most notably former Minnesota governor Tim Pawlenty. It was just Romney’s second public appearance this year in the state that as much as any other will be crucial to his hopes of winning the 2012 GOP nomination.

Romney’s performance, before an audience that included establishment Republicans and grass-roots activists, was sound but hardly dazzling. His stump speech is a work in progress. The health-care plan he signed in Massachusetts remains a potentially big problem. His explanation does not satisfy many of the voters he will be courting in the months ahead.

Public appearances by Romney anywhere this year have been rare. He announced the formation of his campaign committee with a low-key video on his Web site. Major speeches have been fleeting (although advisers say those will come in due time), as have television appearances or interviews. When Romney has wanted to say something about an issue, he has picked the safest of all forums: the op-ed pages of major newspapers.

Events have conspired to draw attention away from Romney. Donald Trump has sucked up just about all the oxygen there is on the Republican side these days, descending from the skies here last week in a helicopter emblazoned with his name, trailed by the media pack. Spectacle becomes Trump and vice versa.

The comings and goings of other GOP candidates have taken whatever other space exists for discussion of the possible challengers to President Obama. Mississippi Gov. Haley Barbour out. Rep. Ron Paul (Tex.) in. Indiana Gov. Mitch Daniels deciding. Etc. Etc.

That has worked to Romney’s benefit. “All of that takes focus off Mitt Romney so that he can just do his own thing,” said Mike Dennehy, a New Hampshire Republican strategist. “Clearly, once that focus shifts away from the entertainment into heavy primary campaign issues, then he’s going to have to start battling.”

Four years ago, Romney was setting a frenetic pace. Then, as a little-known candidate, he was scurrying from state to state, running television ads in Iowa and New Hampshire, and doing everything he could to prove he deserved to be in the same league as the likes of Sen. John McCain (Ariz.) and former New York Mayor Rudy Giuliani.

This year, he runs as if he couldn’t care less what others are doing. He has chosen the issues on which he wants to speak — mostly the economy and national security — rather than feeling the need to respond to the cable catnip of the day. Four years ago his team wanted to win every news cycle. This year they operate with seeming indifference to whatever may be trending politically on Twitter.

Romney has managed to avoid engaging his rivals, meaning he can concentrate his fire on the president. “They haven’t been forced out into the open yet, which I think is unexpected and remarkable,” said Fergus Cullen, a former New Hampshire Republican Party chairman.

When the first GOP debate is held Thursday in South Carolina, Romney is likely to be missing in action. Becoming a punching bag for Ron Paul or some of the marginal candidates who could be in the debate is not in Romney’s playbook for May 2011. “Stay tuned,” he told reporters when they asked Friday whether he would be there.

He might duck the South Carolina debate, but avoiding a scheduled debate here in New Hampshire in June will be harder, given the importance of the Granite State to Romney’s strategy. George W. Bush skipped an early New Hampshire debate in 1999 and, some strategists here say, started to unravel after that and eventually lost the state’s primary to McCain.

Friday’s forum was a benign event: serial speeches by the five prospective candidates, plus two questions for each from a moderator. Still, Romney got a taste of what’s coming — and the way one of his key rivals will deal with him. One of his two questions focused on the Massachusetts health-care plan. “I was hoping I would get that question,” he claimed, too eagerly. “Thank you. Thank you. It’s about time.”

He said there were a lot of problems he tried to solve in Massachusetts: portability, preexisting conditions and the cost to the state of providing care to people without insurance. He said Massachusetts was paying out “hundreds of millions of dollars giving out free care to people who could have afforded to take care of themselves.”

He didn’t say so explicitly, but that is a reason the plan imposes a mandate on individuals to buy insurance — the very provision that is anathema to conservatives in Obama’s plan.

Romney said the plan was not perfect. “Some things worked out; some didn’t,” he said. At no point did he offer regrets. Instead he emphasized that he doesn’t see it as a model for the rest of the country and is committed to repealing Obama’s plan.

That answer contrasted with the more crisp explanation former Minnesota governor Tim Pawlenty gave for his change of position on cap and trade legislation, from once favoring it to now opposing it. “I don’t try to defend it,” he told the audience. “Everybody’s got a couple of clunkers in their record. I just say that’s one of mine. It was a mistake. It was dumb.”

Pawlenty was asked after the event whether he was trying to draw a contrast with Romney over Romney’s failure to apologize for his health-care plan. “I was just referring to my own programs,” he said. He knew others would make the connection, and they did.

Philip Wittmann said he thought all five speakers had done well, though he especially liked Rep. Michele Bachmann (Minn.), former Pennsylvania senator Rick Santorum and businessman Herman Cain. When asked about Romney, he replied, “He’s got a little to overcome with the Massachusetts health-care thing. Pawlenty said, ‘I made a mistake with cap and trade.’ Mr. Romney didn’t say, ‘I made a mistake with the health-care plan in Massachusetts.’ I’d like him to admit it was a big mistake.”

Wittmann is not likely to get that admission from Romney, whose advisers think the answer their candidate gave Friday will satisfy enough Republican voters who otherwise see him as their best hope of defeating Obama to make him the nominee. Others aren’t so sure.

Friday’s forum was the end of the beginning of a strange chapter in the Republican race. Romney has done a good job of keeping his head down while building his fundraising network and quietly staying in touch with supporters from his first campaign. That has served him well. Harder days lie ahead.