MOUNT VERNON, Ohio — Presidential hopeful Mitt Romney deviated Wednesday from his usual well-choreographed campaign rally for a flying-without-a-net Oprah-style town hall in which audience members asked questions. The question-and-answer session at the Ariel Corp. manufacturing plant appeared to be a warm-up for next week’s presidential debate between Republican nominee Romney and President Barack Obama. The debate will use a town-hall format with an inquisitor audience.

Through seven questions from a friendly sparring partner of an audience at the manufacturing plant, which makes natural gas compressors, Romney, clad in jeans and a white shirt, carried a microphone among listeners and touched on the key themes of his campaign .

But one question showed the potential danger of the town-hall format for a candidate. When a man asked Romney whether he would have vetoed “Section 1021 of the 2012 National Defense Authorization Act,” which “gives the president the explicit power to detain, via the armed forces, any person, including us, U.S. citizens, for an indefinite period of time without trial,” the candidate paused.

The questioner was referring to bill passed by Congress and signed into law by Obama in December 2011 that allows the military to detain terror suspects indefinitely without trial, even if they’re U.S. citizens.

Romney said, “I can assure you when I become President ... I will not do things that interfere with the rights of our citizens and their freedom,” then sidestepped whether he would have vetoed the bill if he were president.

“As to that specific piece of legislation, I’m happy to look at it, but I don’t believe that this is the time for us to be pulling back from our vigilance to protecting America and keeping us safe from the kinds of threats we face around the word,” he said.

But when Romney was asked in January, during a Republican presidential primary debate in South Carolina, whether he would have signed the National Defense Authorization Act as written, he responded with a firm, “Yes, I would have.”

“And I do believe that it is appropriate to have in our nation the capacity to detain people who are threats to this country, who are members of al-Qaida,” he said to a smattering of boos from the Myrtle Beach audience.

Romney only occasionally employs the unpredictable town-hall format, preferring to stick to the well-scripted traditional campaign events, where he usually gives a brisk 15- to 20-minute stump speech.

“We’ve done them before, and we view them as a good opportunity to greet voters and talk to them directly about some of the concerns they have about how important issues are affecting their community,” Kevin Madden, a Romney senior adviser, said of the town halls.

Still, opening himself to questions presents some risks for Romney. He is sometimes prone to gaffes or straying off message when speaking off the cuff. Such a moment happened Tuesday in an interview with the Des Moines Register, in which he said regulating abortion would not be part of his legislative agenda if elected president.

“There’s no legislation with regards to abortion that I’m familiar with that would become part of my agenda,” he told the paper’s editorial board.

The Register reported that Romney “by executive order, not legislation,” would reinstate the so-called Mexico City policy that bans U.S. foreign aid dollars from being used to perform abortions. Obama ended the George W. Bush-era policy shortly after taking office.

On the campaign trail and during the Republican primaries, Romney said he is against legalized abortion.

At an Ohio campaign stop Wednesday, Romney told reporters, “I think I’ve said time and again, I’m a pro-life candidate. I’ll be a pro-life president. The actions I’ll take immediately are to remove funding for Planned Parenthood. It will not be part of my budget.”

Some social conservatives have been wary of Romney on the issue because of his previous support for legalized abortion. During a Massachusetts gubernatorial debate in 2002, Romney vowed, “I will preserve and protect a woman’s right to choose.”

As Massachusetts governor, Romney vetoed a bill that would provide access to emergency contraception to women. He declared himself “pro-life” in a Boston Globe opinion/editorial piece in 2005 and defended his conversion on the issue during a Republican presidential primary debate in November 2007.

Obama’s campaign on Wednesday seized upon Romney’s “legislation” remarks, accusing him of trying to hide his “extreme” views in the final weeks before the Nov. 6 election.

“We know the real Mitt Romney will say anything to win,” said Stephanie Cutter, Obama’s deputy campaign manager. “Voters shouldn’t be fooled. ... Women simply cannot trust Mitt Romney.”