Washington (CNN) - Republican Sen. Jim DeMint of South Carolina will resign from his Senate seat as of Dec. 31 to take over as head of the Heritage Foundation, his office announced Thursday.
"I honestly believe that I can do a lot more on the outside than I can on the inside," DeMint told reporters at Heritage Thursday.
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On CNN's "The Situation Room," he explained further: "After this last election it's apparent that we need to do more as conservatives to convince Americans that our ideas and our policies are going to make their lives better. The Heritage Foundation is the premier think tank research organization - the premier idea group for the conservative movement. This will give me the opportunity to help take our case to the American people and to translate our policies into real ideas."
According to a source close to the senator, DeMint was formally offered the job and accepted it on Wednesday. He told his staff Thursday morning and called Senate Minority Leader Mitch McConnell and South Carolina Gov. Nikki Haley around 9 a.m. ET to tell them the news.
Elected in 2004, DeMint was re-elected in 2010. His term was not up until 2016. Haley, a Republican, will name a successor. A special election will be held in 2014 for the last two years of his term.
DeMint, 61, had always planned on leaving after two terms, according to a DeMint adviser.
"This move puts him in a powerful position to further advance fiscal conservative principles," the adviser said.
DeMint, a kingmaker among conservatives, is highly influential and well-beloved in the tea party movement, and has been a thorn in the side of establishment Republicans. In 2009, he was the first to endorse Marco Rubio of Florida in his 2010 Senate bid, at the time that the National Republican Senatorial Committee was backing Florida Gov. Charlie Christ.
He was also a powerhouse in the 2012 election. He held a highly-sought endorsement in congressional races and used his super PAC, Senate Conservatives Fund, to back tea party favorites in GOP primaries. Among his picks were successful Senate newcomers Ted Cruz of Texas, Jeff Flake of Arizona and Debbie Fischer of Nebraska.
"I know I'm leaving the Senate better than I found it with some real leaders," DeMint said Thursday, referring to his role in "stocking the Senate with solid conservatives."
The South Carolina senator also held a strong voice in the Republican presidential primary. In September 2011, he hosted a forum in the Palmetto State for five of the presidential candidates, including Mitt Romney.
However, ahead of the state's first-in-the-South primary in January, DeMint announced he would not be supporting a candidate. "I do not have a favorite in this race and I will not endorse a candidate."
Former House Speaker Newt Gingrich ultimately won the South Carolina contest.
According to a statement from Heritage, DeMint will start as president-elect in early January, then take over in April, replacing the current president, Edwin Feulner.
"Jim DeMint has shown that principled conservatism remains a winning political philosophy. His 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," Thomas Saunders, the group's chairman of the board, said in a statement.
Addressing his new staff on Thursday, DeMint told them he felt like he had "walked into the front door" of his own house and cited Heritage policy papers that first motivated him to run for Congress.
"The ugly duckling looked and saw his reflection and found he was not an ugly duckling at all, but a beautiful swan. Not that I'm a beautiful swan, but I looked at all these papers and realized I was a conservative," he said.
In the recent fiscal cliff debate, DeMint has been a staunch opponent of Democratic-backed proposals to raise tax rates as part of a deficit-reduction solution. He even criticized his own party for ceding ground in the debate by agreeing to raise revenue through tax reform.
"This federal government doesn't need more money," he said, tweaking House Speaker John Boehner's counter-offer this week, which includes $800 billion in new revenue.
In his comments at Heritage, DeMint said "a lot of his role" in the Senate has been "stopping bad things and saying no to bad things."
He added that one of the Republican Party's biggest mistakes in recent years has been "trying to make Obama the issue without sharing with America bold, new ideas."
READ MORE: Who will replace DeMint in the Senate?
All eyes are now on Haley, as the South Carolina governor decides whom to name to fill DeMint's seat. The senator has made it known in Columbia that he wants Rep. Tim Scott to be appointed to his seat, sources tell CNN. If named, he would be the Senate's only African-American member.
But DeMint’s official staff in Washington is pushing back hard against that suggestion, in part because DeMint can’t be seen as meddling in a process controlled by the governor.
Thanking DeMint for his service, Scott said in a statement "Governor Haley will now appoint a new Senator, and I know she will make the right choice both for South Carolina and the nation.”
Rep. Mick Mulvaney is another name being floated. The congressman also said he has "faith Gov. Haley will appoint someone with the character, leadership, and conservatism Senator DeMint has provided South Carolinians for the past eight years."
In a statement Thursday, Haley applauded DeMint's service in the upper chamber, saying he has served "the national conservative movement exceptionally well."
"His voice for freedom and limited government has been a true inspiration," she continued. "On a personal level, I value Jim's leadership and friendship. Our state's loss is the Heritage Foundation's gain. I wish Jim and Heritage all the best in continuing our shared commitment to America's greatness."
Senate Minority Leader McConnell also released a statement, thanking DeMint "for his uncompromising service to South Carolina and our country in the United States Senate."
"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.
DeMint's departure means there will now be 34 (up from 33) Senate seats up for grabs in the 2014 midterm elections, with the Democrats defending 20 of those seats and the GOP defending 14 (up from 13).
South Carolina will be a busy place that year, as Palmetto State voters will cast ballots for two Senate seats: Along with the special election, senior Sen. Lindsey Graham will be up for re-election. A gubernatorial race will also take place.
- CNN's Ted Barrett, Peter Hamby, Paul Courson, Dana Bash, Deirdre Walsh, Dana Davidsen, Shawna Shepherd, and Martina Stewart contributed to this report.