The South Carolina lawmaker said in a statement he was stepping down to become president of the Heritage Foundation. His office said he would take the new job effective Jan. 1.

DeMint was first elected to the Senate in 2004 and easily re-elected six years later. He previously served in the U.S. House of Representatives for three terms.

"I'm leaving the Senate now, but I'm not leaving the fight. I've decided to join The Heritage Foundation at a time when the conservative movement needs strong leadership in the battle of ideas," DeMint said in a statement. DeMint was unavailable for comment, his office said.

Republican Gov. Nikki Haley will appoint DeMint's successor. She didn't immediately signal whom she might choose, or whether she would appoint herself. Haley spokesman Rob Godfrey says the attention should be on DeMint.

"Our state's loss is the Heritage Foundation's gain," Haley said in a statement.

DeMint's former state director, Luke Byars, said the senator's new role would allow him to effect change outside the U.S. Capitol. In the fall election, Democrats strengthened their hand in the Senate.

DeMint, who previously ran a marketing firm, thought conservatives didn't do a good job communicating their message in the presidential race, Byars said.

"He knows how to communicate," said Byars, a political consultant. "This is a vehicle for him to push and pull on conservative issues on a national stage, to get the attention of folks inside Washington. This lets him go back into something he loves. He's always been an idea guy."

The chairman of the state Republican Party said he's stunned, and said that DeMint's resignation will have a ripple effect on state GOP leadership.

Whomever Haley appoints would face a special election in 2014 to finish DeMint's term, which expires in 2016. U.S. Sen. Lindsey Graham faces re-election in 2014.

South Carolina GOP Chairman Chad Connelly said DeMint redefined how the U.S. Senate worked by taking principled stands against the party leadership.

"He's been a conservative rock star," Connelly said. "I'm sure the conservatives in South Carolina will be heartbroken. DeMint's been an icon for a lot of us in the conservative movement."

DeMint mounted his first Senate bid as a conservative with a soft side — he had a memorable commercial where his young adult daughters kept interrupting him to tout his conservative beliefs.

But when he got to the U.S. Senate, DeMint stayed right. He said plenty of times he would rather stand with a committed minority than a big-tent majority.

DeMint put his money where his mouth is, using a political action committee to back conservative Senate candidates like Christine O'Donnell in Delaware and Sharron Angle in Nevada, who were ridiculed for their out-of-mainstream beliefs and soundly beaten.

But DeMint also posted wins. He was one of the first Republicans to back Marco Rubio's successful campaign for the Senate seat in Florida.

DeMint is associated with, but no longer runs, the Senate Conservatives Fund, which ranked sitting senators. He formally cut ties with the political action committee he founded earlier this year.

He helped raise more than $25 million for the fund for the 2010 and 2012 elections.

DeMint has had other successes. He opposed longtime Sen. Arlen Specter of Pennsylvania, just before the veteran Republican, facing a challenge from the right, switched to become a Democrat. DeMint also broke with Senate Minority Leader Mitch McConnell in the Republican leader's backyard to support tea party favorite Rand Paul in the Kentucky Senate primary. Both Paul and Pat Toomey, the conservative who won the GOP nomination in Pennsylvania, were elected in 2010.

In other cases, DeMint's silence has been telling. He pointedly refused to aid once-popular Republican Bob Bennett, a three-term senator who was defeated by conservative voters in Utah's GOP convention.

DeMint's positions have earned him rankings as one of the most conservative senators. He supported partially privatizing Social Security and installing a flat sales tax to replace income taxes.

On Thursday, McConnell thanked DeMint for his uncompromising service in the chamber.

"Jim helped provide a powerful voice for conservative ideas in a town where those principles are too often hidden beneath business as usual," he said in a statement. "There is no question in my mind that he raised the profile of important issues like spending and debt and helped galvanize the American people against a big government agenda."