Updated 12:44 p.m. - South Carolina Sen. Jim DeMint, an influential Republican who has helped prod his party rightward, will step down from his seat in January to become the next director of the Heritage Foundation, a conservative think tank.

DeMint announced Thursday that he planned to succeed Ed Feulner, the longtime head of the Heritage Foundation, next month. The news of his departure was first reported by the Wall Street Journal.

"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. "No organization is better equipped to lead this fight and I believe my experience in public office as well as in the private sector as a business owner will help Heritage become even more effective in the years to come."

The Heritage Foundation said DeMint's "passion for rigorous research, his dedication to the principles of our nation’s founding, and his ability to translate policy ideas into action make him an ideal choice to lead Heritage to even greater success."

Since being elected to the Senate in 2004, DeMint has not-infrequently clashed with Republican leaders on specific pieces of legislation and the overall direction of the party.

His conservative obstinance made him a hero among the party's grassroots. DeMint wielded that influence through his Senate Conservatives Fund, supporting more ideologically pure candidates over candidates deemed more politically-able by Republican Party leaders.

DeMint had even been rumored in 2011 to be thinking of challenging Kentucky Sen. Mitch McConnell for the position of Republican leader, though he eventually declined.

Among DeMint's successes through his Senate Conservatives Fund was the championing of senators like Florida's Marco Rubio, Kentucky's Rand Paul, and Utah's Mike Lee. Among DeMint's failures were the two Senate candidates -- Richard Mourdock in Indiana and Todd Akin in Missouri -- who lost their 2012 bids in the wake of controversial remarks about rape.

"I think if you're interested in having Republicans control the Senate you have to back Republicans who fit their state and who can win in a general election not just in the primary," Maine Sen. Susan Collins, a moderate Republican, told NBC News about whether there were any lessons from DeMint's tenure on Capitol Hill.

J. Scott Applewhite / AP

Sen. Jim DeMint, R-S.C.

DeMint had said he would leave the Senate in 2016, following the completion of his second term, meaning his departure in January will be ahead of schedule.

South Carolina Gov. Nikki Haley, another conservative Republican who enjoys grassroots support, will be allowed to name a replacement, who would have to face a special election in 2014 to serve out the remaining two years of DeMint's term. A variety of Republicans could jockey to replace DeMint, from Haley herself to a couple of her allies -- Tedd Pitts, her deputy chief of staff, or State Rep. Nathan Ballentine -- to U.S. Rep. Tim Scott or other members of the state's GOP establishment.

"Looking forward, Governor Haley will now appoint a new Senator, and I know she will make the right choice both for South Carolina and the nation," Scott Said in a statement.

South Carolina Sen. Lindsey Graham, a relatively more moderate Republican who's expected to face a primary challenge, is also up for re-election that fall. 

There is also the broader question of who, among the 45 Senate Republicans, will seize the mantle of conservative leadership in the upper chamber. Several of the senators whom DeMint helped elect have modeled their actions after the Palmetto State senator, creating their own political action committees and breaking on occasion with the GOP leadership.

NBC's Tom Curry contributed.