McConnell is likely to face Kentucky Secretary of State Alison Lundergan Grimes, Democrats' top remaining recruit, in 2014. Lundergan Grimes announced her bid last week. Her father, Jerry Lundergan, is a well-connected former party chairman, and the family is close friends with the Clintons.

Democrats have set their sights on McConnell as one of their top pickup opportunities in a cycle that offers the party few avenues for offense. They believe he's vulnerable, citing multiple polls that show him to be unpopular in his home state.

But he's been building an operation for months now, and has so far raised $15.4 million for the cycle. If he keeps up this fundraising pace, he could exceed the $21 million he raised for his 2008 race, which he won by 6 percentage points.

President Obama is deeply unpopular in the red-leaning state, losing Kentucky by more than 20 points in 2012.

Lundergan Grimes is not required to report her fundraising until October, after the close of the third quarter.