WASHINGTON -- Sen. Jim DeMint of South Carolina, who in just a few years became a hero to conservative activists nationwide for his vehement anti-government voice and willingness to confront even fellow Republicans, surprised colleagues Thursday by saying he will resign the Senate to run an influential inside-the-Beltway think tank.

DeMint will leave Congress in the next month. This spring, he will take over the Heritage Foundation, a prominent advocacy group with which he has worked closely during his rapid rise from a virtually unknown lawmaker to a conservative kingmaker who bankrolled Senate candidates willing to defy party leaders in order to slash federal spending.His exit frees the Senate Republican leadership from a frequent political irritant. But DeMint’s new role puts him in position to forcefully push the conservative agenda as the GOP undergoes a period of political introspection following its failure to deny President Barack Obama a second term and to take control of the Senate.

“One of the most rewarding things I’ve done in the Senate is work with the grassroots to help elect a new generation of leaders who have the courage to fight for the principles of freedom that make this country so great,” DeMint said in a statement announcing his resignation. “I’m confident these senators will continue the legacy of conservative leaders before them.”

DeMint became known as Senator Tea Party for his hard-ribbed opposition to congressional spending earmarks, immigration reform and the landmark health care bill that Obama signed into law in 2010. He gained an easy re-election to a second term that same year.

DeMint’s passionate following among conservatives across the country helped him become a fundraising giant, whose Senate Conservatives Fund spent more than $25 million to help propel eight candidates to the Senate in the 2010 and 2012 elections.

“Jim DeMint has shown that principled conservatism remains a winning political philosophy,” said Thomas Saunders, chairman of the Heritage Foundation’s board of directors. “This is a crucial moment for America and the conservative movement, and we are seizing it.”

Saunders said DeMint will join the prominent conservative think tank in April, replacing current head Edwin Feulner.

Despite DeMint’s success in getting some of his acolytes elected to Congress, he also drew criticism from Republican leaders and conservative commentators. Many blamed him for backing other candidates whose general election losses helped cost the party a Senate majority in the past two congressional elections.

Republican South Carolina Gov. Nikki Haley will appoint DeMint’s replacement to serve alongside senior Sen. Lindsey Graham. The next statewide election will be held in 2014.

With Haley’s approval rating in her state at 38.3 percent, according to a Winthrop University poll released this week, prominent politicians in South Carolina speculated that she might resign as governor and then be appointed U.S. senator by Lt. Gov. Glenn McConnell, who would replace her as the state government’s top executive.

Haley ruled that scenario out in her public comments Thursday. “No, I will not be appointing myself,” she told a local radio station. “That is not even an option, not something I’m considering at all.”

Haley said DeMint’s “voice for freedom and limited government has been a true inspiration.”