Sen. Jim DeMint, the Tea Party favorite who riled moderate Republicans this week by bashing House Speaker John Boehner's "fiscal cliff" proposal, announced his surprise resignation from the Senate on Thursday.
In a statement, he said he will leave in January to become president of conservative Heritage Foundation think tank.
"I'm leaving the Senate now, but I'm not leaving the fight," DeMint said in his statement. "It's time for me to pass the torch to someone else and take on a new role in the fight for America's future."
The 61-year-old South Carolina Republican was first elected to the Senate in 2004. He previously served in the U.S. House for three terms.
Earlier this week, DeMint denounced the $800 billion in higher tax revenue over 10 years that would come from Boehner's plan, calling it a "tax hike" that will "destroy American jobs and allow politicians in Washington to spend even more" — while failing to reduce the debt. (Read More: Obama Says Tax Rate Cut for Wealthy Possible.)
Boehner's plan was the Republican response to a White House plan last week that includes $1.6 trillion in higher taxes over a decade.
The House GOP counterproposal also included an increase in the eligibility age for Medicare, and lower cost-of-living hikes for Social Security benefits. Overall, the plan is designed to save $2.2 trillion over 10 years. (Read More: How the Plans Differ)
Republican Gov. Nikki Haley will appoint DeMint's Senate successor. She told a Greenville talk radio station she plans to pick someone who will fight for conservative ideas. and wouldn't let the process drag out. Haley didn't specify anyone she favored to replace DeMint, but did take one name out of contention.
"I will not be appointing myself. That's not even an option," Haley told WORD-FM.
Whomever Haley appoints would face a special election in 2014 to finish DeMint's term, which expires in 2016. South Carolina's senior senator, Lindsey Graham, faces re-election in 2014.
"To say I was stunned is an understatement," Graham said.
DeMint's job with the foundation starts Jan. 3, but he won't officially become president until April 3, when founder Edwin Feulner retires, said foundation spokesman Jim Weidman.
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. He once suggested that gays and unwed pregnant women should not teach in public schools.
On Thursday, Senate Minority Leader Mitch McConnell (D.-Ky.) 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.