Story highlights Senate Majority Leader Mitch McConnell has warned Republican donors

Still, the Senate map still heavily favors the Republicans in the 2018 midterms

(CNN) Congressional Republicans' failure to pass a health care bill is starting to hit them where it hurts: Their pocketbooks.

At least $2 million in contributions promised to the National Republican Senatorial Committee have failed to materialize because donors are expressing frustration with the Senate GOP's inability to fulfill their central campaign promise to repeal and replace Obamacare, according to two GOP sources familiar with the matter.

The shortfall, officials said, also points to a larger concern within the party that their core voters may be disillusioned heading into the 2018 midterms when Democrats have a serious shot of retaking the House.

The Senate map still heavily favors the GOP in 2018, but Majority Leader Mitch McConnell has been privately warning his colleagues and donors that the GOP's 52-48 majority should not been taken for granted, sources say. And a funding shortfall could impact the ability of the party's main Senate campaign committee to mount a high-dollar ad blitz in a number of red and purple states where Democratic senators are vulnerable.

A spokesperson for the NRSC declined to comment to CNN.

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