Donors frustrated with Senate Republicans over their inability to repeal ObamaCare are withholding more than $2 million in planned contributions to the National Republican Senatorial Committee, two GOP sources told CNN.

Officials told the network that the GOP is concerned with voters' frustration going into the midterm elections in 2018.

Sources also told CNN that Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE (R-Ky.) has been warning leadership and the party's donors not to take the party's 52-48 Senate majority for granted.

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While Democrats are on the defensive to hold 25 of the 33 Senate seats up for reelection, a serious drop in Republican funding would damage their efforts in vulnerable states.

The Democratic Senatorial Campaign Committee currently holds a financial advantage going into the midterms, CNN noted, with $17.2 million in cash-on-hand as compared with $16.2 million for the Republican committee.

Senate Republicans in July failed to pass a "skinny repeal," a revised and downscaled version of legislation to repeal the Affordable Care Act, casting doubt as to whether they can fulfill campaign promises made over the past several years.

Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) returned to the Capitol after his recent brain cancer diagnosis to cast the deciding vote to kill the legislation. McCain then gave a speech on the Senate floor calling for bipartisan discussion over healthcare reform.

The Senate earlier had narrowly voted to move forward on the debate over healthcare reform, with Vice President Pence breaking a tie on the issue.