Though campaign officials still view Romney as the likely nominee, they have begun to consider the implications of a Santorum victory. They view him as a weaker general election opponent, but one who has shown an ability to connect with the population that is most disillusioned with Obama: white, blue-collar voters.

“Looking at the way the Republican race is unfolding, we’d be derelict if we didn’t begin looking at Santorum as a real potential opponent,” said Stephanie Cutter, deputy campaign manager for the Obama campaign. “We hadn’t been doing that, but circumstances have changed.”

Santorum has appeared increasingly confident of his prospects, campaigning heavily in Michigan — a state that until recently was considered a shoo-in for Romney — and touting his strong poll results in Ohio. Both are crucial swing states with large numbers of manufacturing workers.