Declaring "I'm ready to lead," former Pennsylvania Sen. Rick Santorum announced Monday that he is entering the race for the Republican presidential nomination -- casting his campaign as the scourge of the Obama administration's big-government policies. 

Santorum made it official during a rally in Somerset, Pa. He used the occasion to launch a broadside on President Obama for the state of the economy and the rate of spending under his administration, accusing him of borrowing against future generations to "prop you up" -- blending a fiscal message into the social conservative platform for which he is best known.  

He took particular aim at the federal health care overhaul, calling it an attempt to "hook you." 

"We are facing a time when we have a group of people led by President Obama, who believes that America's greatness lies in its government, not its people," he said. "They want to hook you. They don't want to free you." 

Santorum joins a growing list of Republicans in the race, including former Minnesota Gov. Tim Pawlenty, former Massachusetts Gov. Mitt Romney, Texas Rep. Ron Paul, former House Speaker Newt Gingrich, former New Mexico Gov. Gary Johnson and businessman Herman Cain.

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Other possible but undeclared candidates are Rep. Michele Bachmann of Minnesota, former Alaska Gov. Sarah Palin, former Utah Gov. John Huntsman and Texas Gov. Rick Perry. 

Monday's announcement was not without its interruptions. After a popping sound rang out toward the top of the speech, Santorum assured the crowd, "Those are balloons, not shots." Then he temporarily stopped his speech to point out that somebody else in the crowd needed medical attention, possibly from the heat, and briefly stepped off the stage to assist the person near the front row.

Santorum, the former No. 3 Republican in the U.S. Senate, spoke before the Somerset County Courthouse, a location that his team said is significant because it is near where Santorum's grandfather settled in the U.S. "after leaving fascist Italy to work in the Pennsylvania coal mines until he was 72 years old. He chose to come to America for the freedom our nation offered him."

He said in a television interview earlier that he's "in it to win it."

Santorum will travel next to Iowa for events on June 7, and to New Hampshire on June 8-9. He has already visited dozens of times the three early voting states of New Hampshire, Iowa and South Carolina. He said he doesn't want to appeal to niche groups in the GOP base, but to all comers and that means going to all the key locations. 

Santorum is popular among social conservatives who prefer his strong anti-abortion stance and opposition to gay marriage and embryonic stem cell research

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 sometimes abrasive style alienated voters in Democratic-leaning Pennsylvania, and they replaced him with Bob Casey, an anti-abortion Democrat.

Santorum said Monday that he lost the election because he stood for some unpopular positions, like Social Security reforms that are now no longer the third rail. He said looking back, he may have lost, but he didn't flinch and stood by what he believed in.

Santorum's policy positions align with national conservatives who now are looking at many of the expected candidates with skepticism.

Former Massachusetts Gov. Mitt Romney's changes of heart on gay rights and abortion do little to help his second presidential effort. Former House Speaker Newt Gingrich is twice divorced. Former Utah Gov. Jon Huntsman, who worked for three Republican administrations, nonetheless accepted Obama's offer to be the U.S. ambassador to China.

Santorum, 53, established a presidential exploratory committee to start raising money and joined the first Republican presidential debate in South Carolina. He is expected at next Monday's debate in New Hampshire, having reached the 2 percent threshold required by the hosts to participate.

The Associated Press contributed to this report.