Video

COMMERCE, Mich. — Mitt Romney, who has decried the nasty personal tone of the presidential campaign, seemed to make a joke about President Obama’s birth certificate while speaking to voters here Friday, re-injecting the issue of the president’s birthplace into the campaign.

“Now I love being home in this place where Ann and I were raised, where both of us were born,” Mr. Romney said, standing alongside his wife, Ann, and his running mate, Representative Paul D. Ryan. “Ann was born in Henry Ford Hospital. I was born in Harper Hospital. No one’s ever asked to see my birth certificate. They know that this is the place that we were born and raised.”

Mr. Obama, who was born in Hawaii and shared his birth certificate with the national media, has long been dogged with “birther” rumors, from those who falsely question whether he was actually born in the United States.

Mr. Romney is a Michigan native — his father, George Romney, was the state’s governor — and seemed to be speaking off the cuff. But his campaign quickly scrambled to walk back his comments, saying he was simply sharing his Michigan pride.

“The governor has always said, and has repeatedly said, he believes the president was born here in the United States,” said Kevin Madden, an adviser to Mr. Romney, who served as governor of Massachusetts. “He was only referencing that Michigan, where he is campaigning today, is the state where he himself was born and raised.”

Mr. Romney’s comments, however, seemed a world away from his sentiment last week in Chillicothe, Ohio, when he criticized Mr. Obama, accusing him of running a negative campaign. He was responding, in part, to comments from Vice President Joseph R. Biden Jr., who warned an audience in Virginia that the Romney/Ryan ticket would “put y’all back in chains.”

“His campaign and his surrogates have made wild and reckless accusations that disgrace the office of the presidency,” Mr. Romney said then of Mr. Obama. “Another outrageous charge came a few hours ago in Virginia. And the White House sinks a little a bit lower.”

In his introduction Thursday, Mr. Ryan, too, seemed to emphasize cultural differences with Mr. Obama, taking pains to include his exurban, nearly all-white audience at an orchard and farm here within a circle excluding the president.

“Remember about four years ago when he was talking to a bunch of donors in San Francisco and he said people in states like ours, we cling to our guns and our religion?” Mr. Ryan said, emphasizing the word “ours.” It was a reference to Mr. Romney’s native Michigan and Mr. Ryan’s Wisconsin, but also, it seemed, to differences based on religion and class.

“I just have one thing to say,” Mr. Ryan added. “This Catholic deer hunter is guilty as charged, and proud of it.”

Mr. Obama’s campaign snapped to action, blasting out a critical statement just minutes after Mr. Romney made his comment, while voters were still streaming out of the event.

“Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them,” said Ben LaBolt, a spokesman for Mr. Obama’s campaign, in an e-mail statement. “It’s one thing to give the stage in Tampa to Donald Trump, Sheriff Arpaio, and Kris Kobach. But Governor Romney’s decision to directly enlist himself in the birther movement should give pause to any rational voter across America.”

Though several aides to Mr. Romney emphasized that he has previously stated his belief that the president was born in the United States, and was simply talking about his own local roots, Mr. Romney has long made a point of painting Mr. Obama as somebody who “takes his cues from the Social Democrats of Europe” and who just doesn’t understand what makes this country great.

Some of Mr. Romney’s team have previously gotten in trouble for seeming to question the president’s roots. During the primaries, one of Mr. Romney’s sons, Matt, was first to apologize after making a birther joke at a campaign stop. And on a conference call with reporters last month, John Sununu, a Romney surrogate and the former governor of New Hampshire, said, “I wish this president would learn how to be an American.”

Few voters in the crowd believed that Mr. Romney was simply talking about his Michigan roots.

“It was probably a contrast with Obama’s birth certificate issue, and second, that he was born in Michigan,” said Daryl Pender, 56, who owns a small business in town. “We know the hospital where he was born. There’s no doubt about where he was born.”

She added: “I can’t say what’s fair. But it wasn’t relevant to what I wanted to know. I want to hear about his vision for the future.”

Sylvia Kaponi, 65, a retired Ford Motor employee from Livonia, Mich., said there was no doubt in her mind as to what Mr. Romney was referring to — and she thought it was “great.”

“He was talking about a contrast with the president, and being a hometown guy,” she said. “What I liked was that it was a mild way to touch on a touchy subject with no animosity.”

As Mr. Romney and Mr. Ryan campaign through the Rust Belt, they are especially focused on increasing Republicans’ advantage with white working class voters.

Once a solid element of the Democratic base, the white working class voted for Republican presidential candidates in the past three elections. Republicans captured a House majority in 2010 with a 30-point landslide among this group, according to exit polls.

But one reason that might explain the Romney-Ryan emphasis on Mr. Obama’s cultural differences, as well as ads that accuse the Obama administration for weakening welfare work rules, is that Mr. Romney so far appears not as strong among this group.

According to an NBC/The Wall Street Journal poll this week, Mr. Romney leads white working class voters by 13 points, similar to Senator John McCain’s 12-point advantage in 2008. Because the white working class share of the electorate is declining as other demographic groups, including Hispanics, increase, Mr. Romney needs to improve on Mr. McCain’s results.