Sen. Jim DeMint, the top Tea Party advocate in Congress, announced Thursday that he is resigning to take the helm of the Heritage Foundation, where he said he would use the conservative think tank's pulpit to keep Republican lawmakers from veering to the political center.

The South Carolina Republican's announcement stunned fellow lawmakers and saddened his Tea Party constituents and fans, who for years counted on his unwavering support for fiscally conservative principles, even if it meant bucking his own leadership, which he did often.

DeMint's move alarmed some Republicans, who fear that having a political partisan atop the Heritage Foundation could undercut the think tank's reputation for influential scholarship. He will replace Ed Feulner, president of Heritage since 1977.

DeMint, whose Senate term ends in 2017, insisted he would continue to wield conservative influence, though not from inside the Capitol.

"I'm leaving the Senate now but I'm not leaving the fight," DeMint, 61, said in a statement.

DeMint later told radio host Sean Hannity that his new job would make him more effective in "shaping the conservative message around the country" and allow him to "market the best ideas in the world."

Republicans said DeMint's new role at Heritage will free him from the political constraints of elected office so that he can more actively steer the party to the right.

"He can lead an effort to keep Republican principles and policies on track," Republican strategist Brad Blakeman told The Washington Examiner.

Think tanks like Heritage help shape policy and legislation in Congress, but DeMint will bring his own brand of head-butting politics to the foundation, including a reputation for challenging House Speaker John Boehner, R-Ohio. And even some conservatives fear DeMint's overt partisanship could discredit the prestigious think tank and allow DeMint to attack not just liberals but his fellow Republicans.

"Traditionally, that hasn't been a role that Heritage has played," said Sen. John Cornyn, R-Texas.

One scholar at a rival think tank said DeMint "is certainly going to use his role to beat up on the Republican establishment" but that it won't hurt Heritage's reputation "because everyone perceives Heritage as very partisan."

"People see Heritage as a Republican think tank," the rival scholar said.

Soren Dayton, senior vice president at Prism public affairs and a strategist for Republican John McCain's 2008 presidential bid, agreed.

"We shouldn't mistake Heritage for a think tank any more," Dayton said. "It is an activist organization that attempts to move the debate in the country and the Republican caucuses in the House and the Senate to the right. DeMint probably has the most sophistication of anyone in his generation at this, and it is an elegant way to handle a potentially difficult leadership transition."