The National Republican Senatorial Committee is punishing a Republican advertising firm for its work with the Senate Conservatives Fund, a conservative group that has fostered controversy within the Republican Party by challenging some GOP incumbents.

The firm Jamestown Associates has worked for the SCF on many of its attack ads — and will no longer receive business from the NRSC as a result, the committee confirmed Friday.

The move marks the opening salvo in a power struggle that has been brewing behind the scenes between the national Republican establishment and the SCF, which was founded by then-Sen. Jim DeMint of South Carolina. The group has been especially gung-ho in targeting Senate Minority Leader Mitch McConnell, R-Ky., who faces a competitive re-election bid in 2014.

"For too long, the SCF has been a bully," said NRSC spokesperson Brad Dayspring. "They claim purity and walk around with their nose held higher than everyone else. It's all fake. It's an act. It's time to stand up to the bully instead of running away."

"We are not going to do business with groups or operatives that make disingenuous promises, mislead conservatives or attack Republicans simply to line their own pockets," Dayspring added. "Period."

The SCF dismissed the NRSC's decisions as "a classic example of how Mitch McConnell operates."

"Rather than inspiring people to support something larger than himself, he uses threats and intimidation to destroy everyone who disagrees with him," said SCF's Executive Director Matt Hoskins. "This isn’t what leaders do. It's what bullies do."

"Fortunately, the grassroots in this country are not afraid of Mitch McConnell and his gang of K Street lobbyists," Hoskins added. "He doesn't represent their values and they're ready to fight for something better."

The SCF has devoted hundreds of thousands of dollars so far this year to targeting McConnell in favor of another Republican candidate, businessman Matt Bevin — but its broader national strategy is to turn the Senate more conservative by engaging in open races for safely Republican seats.

In an election cycle when national Republicans are laser-focused on what they consider their best chance to retake control of the Senate, that strategy has profoundly irked many operatives and strategists within the party — while energizing a competing base of conservative donors and activists.