One possible successor is U.S. Rep. Tim Scott of Charleston, the only black Republican in Congress, a scenario DeMint was pushing behind the scenes.

Scott and South Carolina’s other three House of Representatives Republican freshmen, all strong tea party supporters, have looked to DeMint for guidance and had frequent strategy sessions with him since coming to Washington almost two years ago. Reps. Mick Mulvaney of Indian Land and Trey Gowdy of Spartanburg also are viewed as possible DeMint successors.

“His commitment to conservative principles leaves a true legacy, and I have greatly enjoyed getting to know and work with him,” Scott said.

Some leading conservative activists around the country had prodded DeMint to run for president this year, but he said he wanted to remain in the Senate and help elect true conservatives to the upper chamber of Congress. In an interview with McClatchy last week, he declined to rule out a White House run.

DeMint said he’ll leave the Senate early next month, before the start of the next congressional session, an important date because it means, under South Carolina law, that whoever Haley appoints as his replacement will have to run for re-election in 2014 instead of in 2016, when DeMint’s second term would have ended.

DeMint’s job change could mean a nearly sixfold pay increase from his current Senate salary of $174,000. Feulner earned just more than $1 million in 2010, according to the 990 tax form the Heritage Foundation is required to file with the Internal Revenue Service as a non-profit group.

At Heritage, DeMint will have a new pulpit from which to preach his conservative ideals, prod incumbent lawmakers on matters of small government and give voice to promising conservative candidates.

DeMint did just that by aiding Sen. Marco Rubio, a leading 2016 Republican presidential aspirant whom DeMint backed for the Senate in 2010 when other party leaders were supporting then-Florida Gov. Charlie Crist. Rubio was quick Thursday to credit his political patron.

“Sen. DeMint is one of the main reasons why I’m in the U.S. Senate,” said Rubio, a Cuban-American former Florida House speaker. “When I ran for the Senate 2 1/2 years ago, no one thought I had a chance to win. Jim DeMint was the first person in Washington that believed in me and invested in me, and I’m eternally grateful.”

DeMint and Graham have helped make South Carolina a central front in the national battle over their party’s core identity as they cast conflicting votes on immigration reform, various spending measures and Obama’s Supreme Court nominees.

Graham, who sees a bigger role for Congress in federal funding, said he was stunned when DeMint told him Thursday morning he would be leaving the Senate.

“Jim made the Republican Party, quite frankly, look inward and do some self-evaluation,” Graham said on the Senate floor. “Jim is right to say that our debt is unsustainable, that Washington does too much and there’s a better way.”

In an indirect reference to their differences, Graham added: “I think we’ve done a pretty darn good job for South Carolina, at times playing the good cop, the bad cop, but always trying to work together, and what differences we’ve had have been sincere.”

DeMint described the widening divide within the party in his 2011 book, “The Great American Awakening,” saying that he faced contempt from other Republican senators as he tried to curb appropriators’ powers and limit senior members’ clout.

DeMint accused President George W. Bush of spending too much and otherwise abandoning conservative principles, and he detailed clashes with Senate Minority Leader Mitch McConnell of Kentucky, who told him in one encounter: “Jim, you can’t change Washington.”

McConnell papered over their conflicts Thursday in praising DeMint.

“Jim helped provide a powerful voice for conservative ideals in a town where those principles are too often hidden beneath business as usual,” McConnell said. “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.”

DeMint also clashed with Sen. John Cornyn of Texas, head of the National Republican Senatorial Committee, by funding conservatives who ran against Cornyn’s chosen candidates. Cornyn was more muted than McConnell on Thursday.

“I congratulate Jim and look forward to continuing to work with him as we fight for the causes and beliefs that we as conservatives hold dear,” Cornyn said.

DeMint was elected to the U.S. House of Representatives in 1998 and served six years there before winning the 2004 Senate election to replace the retiring Sen. Fritz Hollings, a Democrat.

Andrew Shain of The State newspaper contributed from Columbia, S.C.