Over the course of this presidential campaign, there has been one consistent reservation many Republican voters — and others — have expressed about GOP front-runner Mitt Romney. They question his authenticity. They don’t know if they can trust him. They wonder who he really is.

On Sunday, on a debate stage in Concord, N.H., the former Massachusetts governor may have reinforced those doubts. Twelve hours after skating impressively through another debate with his rivals for the Republican nomination, Romney delivered a far less effective performance.

This was the last opportunity for those trying to slow Romney’s march toward the nomination to go after him, and, unlike in Saturday night’s debate, they did. Rick Santorum, Newt Gingrich and Jon Huntsman all had their moments. Romney parried well at times but just as often came out the lesser from the exchanges.

The most telling moment came early in the debate, which was hosted by NBC’s “Meet the Press” and co-sponsored by Facebook and the New Hampshire Union Leader. Santorum argued that when Romney faced a tough reelection campaign as governor of Massachusetts in 2006, he ducked and decided not to seek a second term. Santorum said that when he faced a difficult challenge that same year, he stood and fought for his Senate seat from Pennsylvania — ultimately losing overwhelmingly. Santorum implied that he was a man of principle.

Romney, who has embraced the persona of citizen politician, businessman and political outsider to draw a contrast with President Obama, protested. He had sought the governor’s office in Massachusetts, he said, to make changes, and after four years he had done so.

“Run again?” Romney asked. “That would be about me. I was trying to help get the state into the best shape as I possibly could, left the world of politics, went back into business. . . . For me, politics is not a career.”

Santorum was incredulous. Gingrich, dropping his let’s-be-nice posture, told Romney to stop with the “pious baloney.”

Set aside the question of whether Romney ran away from a tough reelection race in 2006. The reality is that he chose to run for president in 2008 unencumbered by the burdens of being a sitting governor. He did not return to the private sector. By the early days of January 2007, he was raising millions for his campaign, as Gingrich pointed out.

“You didn’t have this interlude of citizenship while you thought about what to do,” the former House speaker said. “You were running for president while you were governor. You were going all over the country. . . . You then promptly reentered politics. You happened to lose to [2008 GOP nominee John] McCain, as you had lost to [Sen. Edward M.] Kennedy” in a Senate bid in 1994.

Gingrich’s closer went straight to the heart of the questions about Romney. “Just level with the American people,” he said.

There was another exchange with Gingrich near the end of the debate. The former speaker challenged Romney over negative — and, in Gingrich’s estimation, untrue — ads aired by a “super PAC” backing Romney’s candidacy. Gingrich demanded that Romney admit the ads were run by his friends and supporters and were not true.

Romney conceded the first part. “They wouldn’t be putting money into a PAC that supports me if they weren’t people who support me,” he said.

Then Romney said, “With regard to their ads, I haven’t seen them.” He then said he had seen one and recounted the charges it made against Gingrich. He said he thought those were all accurate. “If there’s anything in them that’s wrong, I hope they take it out,” he added. That would come too late for Gingrich, however, whose campaign was badly damaged by those ads in Iowa.

Nothing suggests that the cumulative effect of the two weekend debates is likely to cause a major disruption to Romney’s candidacy for the nomination. He handled some questions well Sunday, particularly one about gay rights, showing that he can think on his feet when challenged.

His campaign is both well stocked and smartly run. He is blessed by relatively weak opposition. With few exceptions, he has been the most consistent performer in debates and on the campaign trail. He is the one candidate who has projected a general election message from the start of his campaign.

He remains strong ahead of Tuesday’s primary in New Hampshire. A big victory here could begin to turn the narrative that he’s a 25 percent candidate who can’t rally a bigger share of the Republican Party. A big victory here also could set him up for success in the more challenging environment of South Carolina.

All of which is why so many Republicans see Romney as their most electable candidate in a general election. They see him running a good campaign, and he is likely to be the party’s nominee to challenge Obama.

Still, the doubts remain, and Romney has not found ways to counter impressions that he is a man of privilege who may not understand the struggles of ordinary Americans. When challenged at an earlier debate by Rick Perry, Romney dared the Texas governor to take a $10,000 bet. On Sunday, he quoted his father as saying, “Never get involved in politics if you have to win an election to pay a mortgage,” which would seem to disqualify middle-class Americans from running for office.

These are small things, but little things can add up. In two focus groups conducted by Peter Hart for the Annenberg Public Policy Center of the University of Pennsylvania, voters have given Romney mixed reviews: They see him as a man of high morals devoted to his family and experienced in the private sector, but also as someone disconnected from their lives by his wealth and background.

Obama’s advisers view Romney through two prisms. Through one they see a formidable opponent who can make the economic argument against the president more effectively than any of the other Republicans running and who has been disciplined in the way he has carried himself so far. Through the other, they see someone whose career in the private sector has left him vulnerable to questions about whether he can truly connect with the independent, middle-class voters who will decide the November election.

Romney’s advisers see it differently. They do not believe this election is about empathy but about effective leadership and results. They see Obama as vulnerable on that front and believe whatever limitations Romney may have will be overridden by the state of the economy and questions about whether the president has the answers to the country’s problems.

Everyone now is focused on the outcome of the Republican nomination. But in a matter of months, if not sooner, attention will shift to the choice in the general election. If he is the nominee, Romney will still have these questions to answer.