The social conservative and devout Catholic is also surging in national polls. In one poll in Ohio - one of the vital Super Tuesday states - Santorum is now beating Romney by 18 points.
Santorum's rise is based on an increasingly powerful blend of social conservatism with a hefty dose of blue-collar economic populism.
Santorum's working-class background is something he repeatedly emphasises on the campaign trail. He even talks up the idea of looking after the poor in troubled economic times.
Romney, who is busy trying to prove himself a fiscal conservative, has suffered an avalanche of scorn for his critical stance against the Government-helped bailout of the car industry at the height of the economic crisis. At the time, Romney said government help would kill America's car industry. Last week, he penned an editorial in the Detroit News slamming President Barack Obama as having sold out Detroit's "Big Three" car companies to their unions.
That is not a popular opinion in Michigan. Most people see the bailout as an unmitigated triumph with some experts saying it may have saved up to 1.5 million jobs in the state. Last week General Motors posted record profits and once again became the world's largest car maker, paying out hefty bonuses to its workers. "Romney is either a bad businessman, a lousy politician, or both," said Peter Cohan, a Forbes magazine columnist and business analyst.
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