COMMERCE, Michigan (AP) — Mitt Romney was on the defensive after raising the discredited rumor that Barack Obama wasn’t born in the United States and therefore ineligible to be president. The Republican candidate jokingly declared “no one’s ever asked to see my birth certificate” as he campaigned near his own Michigan birthplace.

Romney insisted later Friday that 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, which opens Monday and concludes with his formal nomination later in the week.

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.

The flap over the joke 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.

Romney told supporters that he and his wife, 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 an interview with CBS television 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.”

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.

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.

Romney has, however, embraced the support of developer 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.

Obama campaign officials said the comment was evidence that Romney is trying to curry favor with the most conservative Republicans — 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.”

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Associated Press writers Beth Fouhy and Julie Pace in Commerce, Michigan, Jim Kuhnhenn and Kasie Hunt in Washington, Tom Beaumont in Tampa, and deputy polling director Jennifer Agiesta in Washington contributed to this report.