In an August meeting with GOP senators, McConnell urged members to transfer money from their leadership PACs to the Senate Leadership Fund even after a presentation showing that Senate GOP leaders had given far less to the NRSC than top House Republicans have given to their campaign arm, a source familiar with the matter said. Just two GOP senators ponied up to the NRSC after that presentation.

"Well, it concerns me a lot," Texas Sen. John Cornyn, a member of McConnell's leadership team and former NRSC chairman, when asked about the money the committee is burning through. "The Democrats are going to vastly outspend Republicans across the board. But as long as we have enough money to tell our story and to defend our opposition, I think we'll be fine."

Other prominent Republicans have also been taking matters into their own hands. Senate GOP Whip John Thune of South Dakota has been in talks with candidates over setting up joint fundraising committees to help them directly, according to a source familiar with the matter, and has raised concerns with the NRSC's cash problems.