According to the survey, conducted for McConnell by Voter Consumer Research, the Senator has 68 percent support among Republican primary voters to Bevin's 21 percent support. Only eight percent of GOP primary voters are undecided.

Internal polls typically skew in favor of the campaign conducting them, and the result should largely be taken with a grain of salt. The internal poll's outcome does track closely with an earlier GOP poll, which gave McConnell nearly 60 percent support among GOP primary voters.

The polling, however, is the latest indication McConnell isn't taking the GOP nomination for granted. His campaign also launched a new ad this week hitting Bevin for his claimed educational ties to MIT, an attack that's drawn the challenger off-message for the past few days.

Voter Consumer Research surveyed 600 likely voters from Aug. 18-20 and has a margin of error of plus or minus four percentage points.