The former Pennsylvania senator, trying to rally conservative voters and put pressure on Republican rival Mitt Romney, said Obama's 2010 healthcare overhaul gave couples financial incentives to remain unmarried.

"Do you realize that if you are married under Obamacare, you pay a lot more than if you are living together under Obamacare? A lot more," Santorum told 500 voters packed into Froehlich's Classic Corner restaurant. "Thousands of dollars more for the average American family you paid if you are married."

Santorum's attack was the latest in a series of provocative statements in which he or his staff has touched on religion, healthcare or abortion in criticizing the Democratic president.

Over the weekend, Santorum questioned Obama's "theology," and suggested that the president's healthcare law encouraged abortion by requiring insurers to cover various prenatal tests used to identify abnormalities.

Then, on Monday, the former senator's spokeswoman, Alice Stewart, mistakenly referred to Obama's "radical Islamic policies." Stewart later told MSNBC she had meant to say "radical environmental policies."

In calling Obama's healthcare program an enemy of marriage, Santorum apparently was referring to changes in eligibility for Medicaid benefits that can occur when a couple get married.

Obama campaign spokeswoman Lis Smith said the Affordable Care Act passed in 2010 did not create the funding formula Santorum appeared to be complaining about. She added that in many cases, benefits from Medicaid - a program that helps low- and middle-income Americans - actually can increase when recipients get married.

"It's unfortunate that Rick Santorum continues to peddle divisive and patently false claims on the campaign trail," Smith said.

IGNORING ROMNEY

Santorum, after wins in Minnesota, Missouri and Colorado, is suddenly Romney's main challenger in the state-by-state race to determine which Republican will face Obama in the November 6 election.

Recent polls have shown Santorum with a small lead over Romney in Michigan, where Romney grew up and his father was an auto executive and governor.

If Romney, a former Massachusetts governor, loses the Michigan primary to Santorum on February 28, doubts within the party about his campaign will escalate.

After a Sunday evening visit to a Georgia church, Santorum returned to Ohio and Michigan on Monday.

After Michigan votes, Ohio is among 10 "Super Tuesday" states where voters will go to the polls on March 6. The Michigan and Ohio contests will be crucial in helping determine who wins the Republican nomination.

Santorum ignored Romney at the Ohio rally, focusing his attention on the failures of the Obama administration.

On MSNBC on Monday, top Romney adviser Eric Fehrnstrom said Santorum was ignoring economic issues in his campaign.

"Just yesterday, he was talking about World War Two and making a comparison between President Obama and Hitler," Fehrnstrom said. Santorum routinely compares the challenge of removing Obama from the White House with the one that confronted the "greatest generation" during the Second World War.

Asked about it by reporters after his speech on Monday, Santorum said he did not make such a comparison.

"Of course not," he said.

Santorum said his upbringing in nearby western Pennsylvania made him particularly qualified to be president.

"We need someone who understands, who comes from the coal fields, who comes from the steel mills, who understands what average working people of America need to be able to provide for themselves and their families," Santorum said.

(Editing by David Lindsey and Peter Cooney)

• A Permanent Economic Emergency