"He's been a conservative rock star," Connelly said. "I'm sure the conservatives in South Carolina will be heartbroken."
DeMint never was much for compromise in the U.S. Senate. He said plenty of times he would rather stand with a committed minority than a big-tent majority.
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.
DeMint has ties to, but no longer runs, the Senate Conservatives Fund, which ranked sitting senators. He formally cut ties with the political action committee he founded earlier this year.
He helped raise more than $25 million for the fund for the 2010 and 2012 elections.
DeMint also wasn't shy about going after people in his own party if he thought they weren't conservative enough.
He opposed longtime Sen. Arlen Specter of Pennsylvania, just before the veteran Republican, facing a challenge from the right, switched to become a Democrat. DeMint also broke with Senate Minority Leader Mitch McConnell in the Republican leader's backyard to support tea party favorite Rand Paul in the Kentucky Senate primary. Both Paul and Pat Toomey, the conservative who won the GOP nomination in Pennsylvania, were elected in 2010.
DeMint was elected to the U.S. House in 1998 and to the U.S. Senate in 2004, replacing Ernest "Fritz" Hollings. DeMint easily won re-election in 2010 with almost no in-state campaigning. He defeated Democrat Alvin Greene, an unemployed political unknown, with 61 percent of the vote, as he focused on Senate races outside South Carolina.
Graham said the resignation of his friend surprised him.
Graham, who is viewed as more moderate than DeMint, acknowledged his differences with him during comments on the Senate floor, but said they had both represented the state well in different ways.
"I think we've done a pretty darn good job for South Carolina, at times playing the good cop, the bad cop, but always trying to work together," Graham said. "What differences we've had have been sincere, and that's the word I would use about Sen. DeMint. ... We've had a great ride together."
Associated Press Writer Jeffrey Collins contributed to the report.