Sen. Jim DeMint (S.C.) created a new role in the modern Congress: He was a cheerful starter of civil wars -- a Republican who assembled candidates and money to beat other Republicans.

By now, DeMint’s tactics have changed the face of the Senate and made the junior senator from South Carolina into a power broker independent of the party hierarchy. DeMint-backed conservatives such as Rand Paul (Ky.), Mike Lee (Utah) and Marco Rubio (Fla.) won their races and have pulled the chamber’s GOP caucus to the right.

Still, DeMint’s strategy came with serious limitations. And Thursday, some of his closest associates said these may have spurred DeMint’s decision to leave Congress during his second term as a senator. The Senate’s ultimate outsider, they said, may have decided he needed to go farther outside.

“Jim is the guy who’s passionate, [with] conviction, [an] idea guy who has a vision for the country and believes it’s one worth fighting for,” said former senator and presidential candidate Rick Santorum (R-Pa.), who was an ally of DeMint’s in the Senate.

“I think that was his motive to go to the Senate” in the first place, Santorum said. “What this shows is that maybe he’s looking at the Senate as not the best place to do that right now.”

DeMint, 61, is a native of Greenville, S.C., and a small businessman who founded a market research firm in his hometown before he was elected to the U.S. House in 1998. Five years later, he won the Senate seat vacated by retiring Democrat Ernest F. Hollings, a change that reflected South Carolina’s broader political shift from bastion of the Democratic “Solid South” to epicenter of a deep-red modern conservatism.

Out of that environment, DeMint became one of the first elected officials to channel the money and energy of the Tea Party, rising as the movement’s elder statesman in Washington. Along with Paul and Lee, DeMint was a founding member of the Senate Tea Party Caucus.

In the Senate, DeMint has been distinguished by his strident attacks on Democrats: The health-care debate, he predicted, would be President Obama’s “Waterloo.” DeMint said Obama was the “world’s best salesman of socialism.”

But many of DeMint’s most important confrontations have been with Republicans. Under President George W. Bush, he helped derail an immigration proposal supported by Bush and Sen. John McCain (R-Ariz.). He also harangued his fellow Senate Republicans to support a moratorium on earmarks.

Rep. Mick Mulvaney (R-S.C.), a conservative who said he considered DeMint a mentor, said the senator told him not to be co-opted in Washington. “ ‘Be true to yourself,’ ” Mulvaney said DeMint told him. “And he said, ‘You’re going to have to try and figure out if you’re going to fix this from within the system or without the system.’ ”

By “system,” Mulvaney said, DeMint meant the Republican mainstream.

DeMint’s political action committee, the Senate Conservatives Fund, drew big donations from hedge funds, manufacturers and the well-known conservative donors at Koch Industries. In all, it gathered 4,740 contributions of more than $200, according to the Center for Responsive Politics. The PAC raised money — $13.7 million in 2012 — for a string of long-shot conservative candidates, who often defeated mainstream Republicans in party primaries. DeMint’s money helped create a new brand of Republican senator, focused, like DeMint, on shrinking the federal government, cutting spending and supporting conservative social causes.

But in general elections, DeMint’s people lost more often than they won. In 2010, he helped nominate three Republicans — Sharron Angle in Nevada, Christine O’Donnell in Delaware and Ken Buck in Colorado — for Senate seats that had seemed within reach. All three lost their races. In 2012, it happened again, with candidates Josh Mandel in Ohio and Richard Mourdock in Indiana losing to Democrats.

These losses were cold comfort to Republicans whom DeMint had helped undermine before, such as former South Carolina congressman Bob Inglis. Inglis had been a friend of DeMint, but DeMint declined to endorse him in a 2010 Republican primary fight against a former prosecutor, Trey Gowdy, whose politics were closer to DeMint’s.

That strategy may work in South Carolina, Inglis said Thursday, but it is dangerous to attempt on the national level. “We’ve got to get into the business of addition and multiplication, and out of the business of subtraction and multiplication,” Inglis said. “Because it ain’t working out for us.”

The defeat of DeMint’s allies this year helped keep Republicans as the Senate minority and left DeMint with little trust from the GOP leadership. During DeMint’s eight years in the Senate, none of the bills he has authored have been signed into law.

“He hasn’t been terribly successful as a politician or as a senator,” said Mark Tompkins, a political science professor at the University of South Carolina who has followed DeMint’s career. “At the end of the day, he just wasn’t temperamentally a man of the Senate.”

DeMint will move to head the Heritage Foundation, a place where he has deep ties. In fact, in 2009, the head of that think tank said of DeMint: He “may be the junior senator from South Carolina, but here we call him the senior senator from the Heritage Foundation.”