Romney touches 'birther' issue he avoided before

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.

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.

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