The Senate Conservatives Fund, which is trying to oust Senate Minority Leader Mitch McConnell in the 2014 elections, on Friday said the Kentucky Republican is to blame for the GOP's failure to take control of the Senate in 2012.

Matt Hoskins, the fund's executive director, said Republicans remain in the minority in the upper chamber not because voters are rejecting conservative Tea Party candidates, but because establishment Republicans like McConnell are discouraging more conservatives from running.

"Mitch McConnell financially supported and helped nominate at least six moderate Republicans who cost Republicans seats [in 2012]," Hoskins said. "If Mitch McConnell wants to blame someone for the party's failure to win a majority, he should look in the mirror."

Conservatives are challenging McConnell in a 2014 Republican primary in hopes of replacing the Senate's top Republican with a more conservative candidate, Matt Bevin.

The Senate Conservatives Fund, established by former Sen. Jim DeMint to support conservative candidates, released a new ad attacking McConnell on Thursday and said it would spend about $340,000 to air it on Kentucky television. The ad criticizes McConnell for not acting to defund Obamacare.

McConnell's campaign reacted to the ad by accusing the conservative group undercutting GOP chances of taking the Senate.

"There are few organizations in American politics more responsible for the Democratic majority in the U.S. Senate, and thus the continued existence of Obamacare, than the Senate Conservatives Fund," McConnell's campaign said Thursday.

McConnell's bid for a fifth term is widely considered a toss-up, and he faces challenges from both flanks. Bevin, a businessman, is challenging McConnell in a Republican primary, and Secretary of State Alison Lundergan Grimes, a Democrat, is set to challenge the primary winner next November.