U.S. Senator Jim DeMint, a South Carolina conservative and favorite of the Tea Party wing of the party, will resign from the Senate, his office said on Thursday.
DeMint, 61, made waves this week by condemning the offer of House of Representatives Speaker John Boehner, a fellow Republican, to resolve the "fiscal cliff" stand-off.
DeMint will leave the Senate, which Democrats control, in January to run the Heritage Foundation, a conservative think tank in Washington.
This week DeMint said Boehner's offer for $800 billion of increased tax revenues would "destroy jobs and allow Washington politicians" to swell, not reduce, federal deficits.
Boehner made the bid in negotiations with Democratic President Barack Obama to avert the $600 billion in taxes and spending cuts that will start to take effect in January if no deal is reached.
DeMint said he believes the Heritage Foundation will allow him to more broadly popularize conservative views.
"This is an urgent time," DeMint told the Wall Street Journal in an interview before the resignation announcement, "because we saw in the last election we were not able to communicate conservative ideas that win elections."
DeMint was elected in 1998 to the U.S. House, after vowing to serve only three terms. He won election to the Senate in 2004, and was re-elected in 2010.
South Carolina is a solid Republican state and so the retirement is not likely to significantly alter the balance of power in the Senate, where Democrats will control 55 of the 100 seats in January.
South Carolina's Republican governor Nikki Haley will appoint a replacement through 2014, after which there will be a special election.