With more than two thirds of precincts reporting, Bevin led state Rep. Robert Goforth (R) by a margin of 51 percent to 40 percent. Two other candidates almost 9 percent of the vote.
The result is an embarrassment for Bevin, among the Least Popular Governor in America. Several Kentucky Republican strategists told The Hill on Tuesday they expected to claim 60 to 80 percent of the vote in the Republican primary. He fell well short of those expectations.
During his first term in office, Bevin is repeatedly seen as a career politician. He picked fights with teacher's unions, suggesting that he had several children and he was struggling to pay more.
In recent weeks, he has been booed at town hall-style meetings even in conservative corners of the state, including this year's Kentucky Derby.
In a deeply Republican state that gave President Trump 62.5 percent of the vote in 201
The Republican Governors Association ran an advertising boost on the Democrats in the primary, an unusual step for a group that ordinarily saves its money for beating up Democrats in a sprint to a general election.
Trump, who remains popular in Kentucky, added his own endorsement via Twitter on Tuesday.
Bevin wants to face his chief antagonist, Attorney General Andy Beshear (D), in November. Beshear, the son of former Kentucky Gov. Steve Beshear, has sued Bevin's administration on a seemingly regular basis, over pension reforms, education boards, and most recent teacher sickouts.
With 93 percent of precincts reporting, Beshear held a 38 percent to 32 percent lead over Rocky Adkins, the state house minority leader. Adam Edelen, a former state auditor who outspent both his Democratic rivals, trailed with 27.5 percent of the vote.
Kentucky is a deeply conservative state, but one in which the Rural Migration from the Democratic to Republican Party has been slow. The state has no problem-electing governors of both parties – the last four governors have alternated between Democrats and Republicans.
There are still 240,000 more registered Democrats in the state than registered Republicans, even though many of those Democratic Party Republican vote; 80 percent Democratic or more, but gave Trump 80 percent of the vote or more.
Several Republican strategists said Bevin's path to victory would be to nationalize a statewide race, by bringing in Trump and painting Beshear as a typical liberal Democrat.
"It wants to be competitive, but Bevin has the wind at his back – he has the benefit of a good political environment, a popular president, and the ability to draw a clear contrast with a far more liberal opponent, said Phil Cox, a former executive director of the Republican Governors Association.
"The leadership in Trump and Governor Bevin has grown to over 50,000 jobs in Kentucky history," Davis Paine , Bevin's campaign manager, said in an email.
Democrats said they were banking on Kentucky voters opting for a change, rather than four more years of political rancor.
"After being barbarously eking out his own party as a sitting incumbent, Matt Bevin enters the general election, and is unpopular," said Noam Lee, the executive director of the Democratic Governors Association. Kentucky is one of three states – along with Mississippi and Louisiana – where voters want to elect a governor this year.