COMMERCE, Mich. (AP) — Wading into a debunked conspiracy theory, Mitt Romney raised the issue of President Barack Obama's citizenship Friday by joking that "no one's ever asked to see my birth certificate."

At a rally in the suburbs of Detroit, Romney told a crowd of about 5,000 that he and his wife were happy to be back near their childhood home. "They know that this is the place that we were born and raised," the candidate said.

The remark was a clear reference to the discredited claims that Obama was not born in the United States and thus ineligible to be president. Hawaii officials have repeatedly verified Obama's citizenship, and courts have rebuffed lawsuits over the issue. The Obama campaign decried the remark, saying Romney was embracing "the most strident voices in his party instead of standing up to them."

Top Romney adviser Kevin Madden tried to walk the comments back shortly after, saying: "The governor has always said, and has repeatedly said, he believes the president was born here in the United States."

Madden said Romney did not need to apologize because he was simply drawing attention to the fact that Michigan, where he was campaigning, was the state where he himself was born and raised.

But Romney's comments overshadowed an enthusiastic rally where he otherwise devoted himself to criticizing Obama's economic record and laying out a prescription of fiscal discipline, regulatory overhaul and freer markets to create more jobs and energize the private sector. It also gave Democrats, for the second day in a row, an opportunity to jump on Romney remarks that distracted from the run-up to the Republican convention next week.

The birth certificate comment for the moment aligned Romney with a conservative fringe that has pursued the issue. Among the most prominent Romney backers who have questioned Obama's birth in Hawaii have been developer Donald Trump.

"Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them," said campaign spokesman Ben LaBolt. "Gov. Romney's decision to directly enlist himself in the birther movement should give pause to any rational voter across America."

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Madden said Romney wasn't intentionally making a reference to the questions about Obama's birth certificate.

Romney was campaigning with his running mate, Paul Ryan, and with his wife Ann in Michigan, a state where Obama has been leading but where Romney's father, George, made his name both as a top auto executive and as governor.

Obama has been dogged throughout his presidency by question about his birthplace. He released a long form copy of his birth certificate last year, showing he was born in Hawaii in 1961.

Romney has been careful to steer clear of the birth certificate issue, even while enthusiastically accepting support from Trump. Whenever he was asked about the issue in interviews, Romney always demurred and said it was a settled issue.

Among a segment of the conservative, anti-Obama movement, the issue is a rallying cry that continues to persist despite evidence to the contrary.

For Romney, raising the issue at this point runs contrary to his campaigns goal of boosting his support among independent and undecided voters, for whom Obama's birth certificate is presumably a non-issue. It also overshadowed efforts by Romney advisers to establish convention themes that cast him as a compassionate and serious presidential candidate by featuring Romney's personal side and life's experiences.

Romney's birth certificate crack comes a day after he Romney gave Democrats another opening by declaring that big businesses in the U.S. were "doing fine" in the current strapped economy in part because they get advantages from offshore tax havens.

His comments echoed similar assertions by Obama about the state of the private sector, comments Romney has criticized. They're also a reminder that the GOP candidate himself has kept some of his personal fortune in low tax foreign accounts, including Switzerland and the Cayman Islands.

"Big business is doing fine in many places," Romney said during a campaign fundraiser Thursday. "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."

Romney's assertions resembled Obama's declaration earlier this summer that the "private sector is doing fine." Romney and other Republicans pounced on the president's comments and cast them as an indication that he was out of touch with the nation's economic struggles.

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 Mitt Romney.

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 became 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 President Barack Obama.

Santorum won a total of 245 delegates during the primary, according to a count by The Associated Press.