Former Sen. Rick Santorum (R-Pa.) has officially formed a presidential exploratory committee, allowing him to participate in Thursday's presidential debate in South Carolina.

Santorum made the announcement Tuesday afternoon in an interview with conservative radio host Sean Hannity, taking another step in the direction of a formal run for president next year.

Tuesday was the deadline imposed by organizers of Thursday's debate for potential participants to meet the criteria laid out by Fox News Channel and the South Carolina GOP. Participants had to either register a presidential exploratory committee or have announced a formal campaign for president by May 3. 

Last week, Santorum hinted that he was moving in the direction of a formal committee, telling reporters he didn't consider filing with the Federal Election Commission to be "a particularly big step at this point." Earlier this month, the former senator created a "testing the waters" account, which allowed him to start raising money for a presidential campaign.

Santorum said Tuesday that while it's not an official announcement, "I really feel like I'm being called to do this out of service for my country."

He also touted his repeated visits to the early states of Iowa, New Hampshire and South Carolina, while questioning why other likely 2012 candidates have not spent more time on the campaign trail.

Last week in Washington, Santorum offered a blistering critique of President Obama's foreign policy in a speech at the National Press Club, setting the stage for a likely run.

Labeling the president an "apologist" who has fostered "confusion and doubt" on the world stage, Santorum said Obama isn't qualified to advance American interests because he rejects the idea of American exceptionalism.

"When he speaks of our greatness as a country, he ties it to our modern social welfare programs," Santorum said of Obama. "A president who doesn't understand the greatness of the American experiment cannot confidently advance her interests."