Bevin had questioned McConnell's conservative credentials, telling MSNBC that the five-term senator "just pretends to be one every six years in order to try to trick his way back into the U.S. Senate."
Despite financial support from tea party groups, Bevin's campaign failed to out-raise McConnell, who spent $11 million on the campaign. Bevin, who raised $3.3 million, was bogged down by several communication missteps and controversies.
In April, for example, he had to apologize for speaking at a March cockfighting event.
McConnell will take on Democratic nominee Alison Lundergan Grimes, Kentucky's secretary of state, in what's expected to be a hotly contested November election.
McConnell has held his Senate seat since 1985, but he faces stiff opposition from Grimes -- they're virtually tied in recent polls. Though it's unusual for a sitting incumbent to lose his seat, McConnell is one of nation's least popular senators.
McConnell is likely to link Grimes to the president and his controversial health care law, which has proven incredibly unpopular with Kentuckians.
If Grimes wins, she would be the state's first-ever female senator.