Once the No. 3 Republican in the ranks of the Senate GOP leadership, Santorum charged that Obama has worked to undermine Americans' liberties and has imposed a national health care plan that saps individual choice. He accused Obama of spending billions of dollars that will add to the debt of future generations and said the president lacks faith in the nation's potential.
"I'm ready to lead. I'm ready to do what has to be done for the next generation, with the courage to fight for freedom, with the courage to fight for America," Santorum said, speaking the sun-splashed steps of a county courthouse in western Pennsylvania. "That's why I'm announcing today that I'm running for president of the United States of America."
Santorum, who enjoys strong support from the anti-abortion rights bloc in the Republican Party, nodded to the social conservatives who have huge sway in early nominating states of Iowa and South Carolina. He also pitched himself to tea party-style activists who have yet to jell around a single candidate.
"The principal purpose of America was to make sure each and every person was free. Ladies and gentlemen, that is at stake now," Santorum said, pointing to a Democratic-pushed health care law that conservatives loathe.
"Every single American will be hooked to the government with an IV," Santorum said.
"They want to hook you. They don't want to free you. They don't want to give you opportunity. They don't believe in you. ... This president does not trust you to make a decision on your health care plan."
He also said that Democratic spending has put the country on a dangerous path, and Santorum blamed Obama for an economy that collapsed in 2008 before he won election.
"If you look at the record of spending under this president, sure he came in with a problem .... but he kept digging and digging," Santorum said.
In an announcement speech near the coal fields where his immigrant grandfather toiled, Santorum praised the nation's founding fathers and said the nation needs to return to the potential that lured his grandfather from Italy in 1927.
"If they work hard, they can succeed. That's the America my grandfather came to. That's the America my dad lived in," Santorum said. "That's the America we need again today."
Santorum enters the race four days after former Massachusetts Gov. Mitt Romney formally declared his candidacy and former Utah Gov. Jon Huntsman and Minnesota Rep. Michele Bachmann are strongly weighing bids.
Former House Speaker Newt Gingrich, former Minnesota Gov. Tim Pawlenty and businessman Herman Cain are already in a race that has seen some of its biggest names decide against bids. Former Arkansas Gov. Mike Huckabee, Mississippi Gov. Haley Barbour, Gov. Mitch Daniels of Indiana and entrepreneur-entertainer Donald Trump have said they're not running.
There also has been speculation that former New York Mayor Rudy Giuliani, who sought the nomination in 2008, is considering making another run.
Santorum, a lawyer by training, had been laying the groundwork for a presidential bid when he lost a bruising re-election bid to the Senate in 2006. His opposition to abortion, gay marriage and embryonic stem cell research makes him an appealing candidate for conservatives. But his sometimes abrasive style alienated voters in Democratic-leaning Pennsylvania, and they replaced him with Bob Casey, an anti-abortion Democrat.
Earlier this year, he established a presidential exploratory committee to start raising money and joined the first _ though ill-attended _ Republican presidential debate in South Carolina. He is expected at next Monday's debate in New Hampshire, which is likely to include more of the expected field.
Santorum, 53, is married with seven children. He has been out of elective office since 2007 and lacks the robust fundraising or personal wealth of his likely rivals.
But he is a tough campaigner, unafraid of fiery rhetoric.
"I believe now, that Americans now are not looking for someone they can believe in," Santorum said in a mocking reference to Obama's 2008 campaign slogan, "Change You Can Believe In."
"They're looking for a president who believes in them," Santorum said.