COMMERCE, Michigan — 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 US. 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. — AP