Whether he meant to or not, Mitt Romney on Friday injected the toxic issue of birtherism into an already bitter presidential race at an unhelpful time for the presumptive Republican nominee.

Noting that he and his wife, Ann, were born in Michigan hospitals, Romney said at a rally here: “No one’s ever asked to see my birth certificate. They know that this is the place where both of us were born and raised.”

The crowd of several thousands responded knowingly with loud cheers, and several supporters said in interviews following the rally that they interpreted Romney’s comment as a dig at President Obama — although Romney said later it was “not a swipe.”

Although Obama has released his birth certificate showing he was born in Hawaii, some conservative activists still question his citizenship. Romney’s campaign aides quickly rejected that interpretation, insisting that Romney was not referring to “birther” theories and noting that he has been on the record saying he considers Obama’s citizenship a settled matter.

“The governor has always said, and has repeatedly said, he believes the president was born here in the United States,” Romney adviser Kevin Madden said. “He was only referencing that Michigan, where he is campaigning today, is the state where he himself was born and raised.”

The Obama campaign seized on the remark to argue that Romney was embracing “strident” conservatives, including real estate mogul Donald Trump as well as Maricopa County Sheriff Joe Arpaio of Arizona and Kansas Secretary of State Kris Kobach, two of the most hard-line opponents of illegal immigration.

“Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them,” Obama spokesman Ben LaBolt said in a statement. He added, “Governor Romney’s decision to directly enlist himself in the birther movement should give pause to any rational voter across America.”

In an interview with CBS News later in the day, Romney said his remark was “not a swipe.”

“I’ve said throughout the campaign and before, there’s no question about where he was born,” Romney said in the interview. “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 Commerce rally marked the start for Romney of two days of campaigning with his running mate, Rep. Paul Ryan (Wis.), leading into the Republican National Convention, which is scheduled to open in Tampa on Monday.

The rally, staged around two red barns on a sprawling farm on the outskirts of Detroit, got off on a sharp tone, with both candidates delivering biting and at times personal attacks on Obama.

Ryan, who introduced Romney, brought up Obama’s comment about guns and religion in a 2008 fundraiser. “Remember about four years ago when he was talking to a bunch of donors in San Francisco, and that he said that people from states like ours, we like to cling to our guns and our religion?” Ryan said. “I just have one thing to say: This Catholic deer hunter is guilty as charged and proud of it.”

Ann Romney spoke just moments before her husband made the birth certificate comment, talking about how good it feels for the couple to be back in Michigan.

“Mitt and I grew up here, we fell in love here, and this is a special place for us, and we want to have a big ‘W’ next to Michigan in November,” she said.

After she turned the microphone over to her husband, Mitt Romney also was nostalgic.

“I love being home in this place where Ann and I were raised, where both of us were born,” he said. “Ann was born in Henry Ford Hospital. I was born in Harper Hospital. No one’s ever asked to see my birth certificate. They know that this is the place that we were born and raised.”

Romney actually did release his birth certificate in May.

During spring 2011, when Trump was questioning the authenticity of Obama’s birth certificate, Romney dismissed the theories.

“I think the citizenship test has been passed,” Romney told CNBC in April 2011.

Several voters interviewed after Romney and Ryan ended their rally Friday said they interpreted Romney’s comment as targeting Obama.

Susan Cardel, 51, said she thought Romney’s comment was “hysterical.”

“That is still a big issue, isn’t it?” Cardel said. “I think it is, that Obama wasn’t born in America. I think Romney was just making a statement. He wasn’t trying to be mean or anything.”

Jim Gorcya, 72, said he read Romney’s comment as a suggestion that Obama’s birth certificate is not real.

“Obama doesn’t have one,” Gorcya said. “He wasn’t born here, and he can’t find his.”

Nancy Perkins, 57, a retired teacher from Waterford, Mich., said she did not find Romney’s comment funny and said she had no doubts that Obama was born in the United States.

“I’m sure that this was all researched before he was allowed to become president,” Perkins said.

Later Friday afternoon, the Obama campaign appeared to make light of Romney’s remarks in a message sent from its Twitter account.

“Song of the day: Born in the U.S.A.,” the campaign tweeted, along with a link to a YouTube video of Bruce Springsteen performing the song.