COMMERCE, Mich. — Republican Mitt Romney raised the discredited rumor that President Barack Obama wasn’t born in the United States, jokingly declaring “no one’s ever asked to see my birth certificate” as he campaigned Friday near his own Michigan birthplace.

Romney later insisted the remark was just a joke and not meant to question Obama’s citizenship. But the comment risked creating an unwanted distraction for Romney in his last few days of campaigning before the Republican National Convention begins Monday. It came a day after Romney caused another stir by declaring that big business was “doing fine” in the current struggling economy in part because companies get advantages from offshore tax havens.

Romney made his birth certificate remark at a large outdoor rally in Michigan, where he grew up and where his father, George Romney, served as governor. He was joined onstage by his wife, Ann, and running mate, Wisconsin Rep. Paul Ryan.

Romney told supporters that he and Ann had been born at nearby hospitals.

“No one’s ever asked to see my birth certificate. They know that this is the place that we were born and raised,” Romney said.

The crowd of more than 7,000 responded with hearty laughter.

But Obama campaign spokesman Ben LaBolt swiftly denounced the remark, saying Romney “embraced the most strident voices in his party instead of standing up to them.”

Romney was asked about his comment in a CBS interview later in the day.

“No, no, not a swipe,” Romney said. “I’ve said throughout the campaign and before, there’s no question about where he was born. He was born in the U.S. This was fun about us, and coming home. And humor, you know — we’ve got to have a little humor in a campaign.”

The authenticity of Obama’s birth certificate has been questioned by some Republican critics who insist Obama is not a “natural-born citizen” as required by the Constitution. Obama released a long-form version of his birth certificate last year as proof that he was born in Hawaii in 1961. But polls show some Republicans remain unconvinced. A Pew Research Center poll taken in April found 19 percent of Republicans, when asked where Obama was born, said they weren’t sure, and 6 percent believe he was born in another country.

Friday’s remark came as top Romney advisers were announcing convention themes designed to feature Romney’s personal side and life experiences as he introduces himself to a broad national audience with many who have yet to tune in to the presidential contest. It’s also an opportunity for Romney to cast himself as a compassionate and serious candidate for the presidency after a summer of unforced errors and tough Obama campaign ads that have portrayed him as an out-of-touch multi-millionaire.

The joke also threatened to undercut Romney’s recent complaints that Obama has been the one to inject a corrosive tone into the campaign.

Romney gave Democrats another opening Thursday when he attempted to sympathize with the struggles of small business owners.

“Big business is doing fine in many places,” Romney said during a campaign fundraiser. “They get the loans they need, they can deal with all the regulation. They know how to find ways to get through the tax code, save money by putting various things in the places where there are low tax havens around the world for their businesses.”

His comments resembled Obama’s declaration earlier this summer that the “private sector is doing fine” — a remark that Romney and other Republicans pounced on to portray the president as out of touch with the nation’s economic pain. By invoking tax havens, Romney also drew indirect attention to the fact that he has kept some of his own personal fortune in low-tax foreign accounts, including in Switzerland and the Cayman Islands.

Romney’s joking flirtation with the “birther” rumor was a departure for the former Massachusetts governor, who has largely steered clear of the controversy and has said when asked that he believes Obama was born in the U.S. Those around him have sometimes brought up the issue.

Romney’s son Matt apologized earlier this year for saying that his father would release more of his tax returns “as soon as President Obama releases his grades and birth certificate.”

Romney has also embraced the support of Donald Trump, who aggressively questioned Obama’s place of birth during his own flirtation with a presidential run. It was Trump who was the impetus for Obama releasing his long-form birth certificate.

Trump has hosted a high-dollar fundraiser for Romney and is expected to have a role at the Republican convention.

Romney adviser Kevin Madden stressed that Romney has said he believes Obama was born in the U.S. and that his view had not changed.

“He was only referencing that Michigan, where he is campaigning today, is the state where he himself was born and raised,” Madden said.

Obama campaign officials said the comment was evidence that Romney is trying to curry favor with conservatives — some of whom remain unenthusiastic about Romney’s candidacy.

“Governor Romney’s decision to directly enlist himself in the birther movement should give pause to any rational voter across America,” LaBolt said.

Obama’s campaign tweeted from the president’s Twitter account: “Song of the day: Born in the USA” along with a link to the Bruce Springsteen song.

And the Obama camp promptly held up Romney’s remark in a fundraising email to supporters: “Take a moment or two to think about that, what he’s actually saying, and what it says about Mitt Romney. Then make a donation of $5 or more to re-elect Barack Obama today.”

Obama was at the White House Friday with no public events and was headed to Camp David for the weekend.

Meanwhile, in a sign Romney was further solidifying his base, former Republican presidential candidate Rick Santorum said he was releasing the delegates he won during the GOP primary race to the presumptive nominee.

A senior Santorum aide said the former Pennsylvania senator informed supporters of his plans during a Thursday night conference call. He’s expected to release a public statement Friday followed by a formal letter. The aide spoke on the condition of anonymity because he wasn’t authorized to speak publicly.

The decision is a formality that frees more than 200 delegates to support Romney at the party’s national convention next week in Tampa, Fla. Santorum was Romney’s top opponent for the GOP nomination. His withdrawal from the race in April cleared the way for Romney’s general election fight against Obama.