The political arm of United Mine Workers of America, a union of coal miners, has endorsed Sen. Rob Portman (R., Ohio) for reelection, dealing a blow to Democratic challenger Ted Strickland.

The move represents a departure for the union, which has a history of endorsing Strickland, a former governor, and fellow Democrats. Strickland has come under fire in recent months for his ties to anti-coal organizations. Portman’s campaign announced the endorsement from the union’s National Council of Coal Miners Political Action Committee (COMPAC) in a statement Thursday.

"We very much appreciate the support you have given both active and retired coal miners and their families, especially in such difficult times as the coal industry is experiencing today," COMPAC International President Cecil Roberts and International Secretary-Treasurer Daniel Kane wrote in a letter to Portman.

The leaders said that COMPAC will "be actively working among our membership and our communities to inform working and retired Ohioans of our support for you in this year’s election."

The announcement could come as a surprise to Strickland’s campaign, as the union endorsed Strickland for his successful gubernatorial bid in 2006 and his unsuccessful reelection campaign in 2010. Strickland represented Ohio’s coal country during his time serving in Congress in the 1990s and early 2000s.

The union also endorsed all Democrats running in Ohio during the 2006, 2010, 2012, and 2014 election years.

But United Mine Workers of America indicated earlier this year that the union might stay out of the senate race in Ohio altogether after Strickland received scrutiny for his work at a liberal think tank with a marked anti-coal agenda.

Before launching his senate bid, Strickland earned a quarter million dollars heading the lobbying arm of the Washington, D.C.-based Center for American Progress, which pushes for progressive environmental policies. Strickland worked alongside former EPA administrator Carol Browner, who helped architect President Obama’s climate change policies, during his time there.

"Ted Strickland likes to remind people that he’s from southeast Ohio, but he has a record of turning his back on Coal Country. Even though coal is a proven source of relatively inexpensive energy that supports thousands of jobs across Ohio, and even though Ohio relies on coal for approximately 70 percent of our electricity, Ted worked on behalf of a liberal special interest group in Washington that is dedicated to ending coal jobs," Portman said in a statement Thursday.

Strickland has also done little to distance himself from Democratic presidential candidate Hillary Clinton after she angered pro-coal groups by saying that her administration would put coal miners and companies out of business during a town hall event in Columbus in March. Strickland called the comments "unartful" in an interview with the Washington Free Beacon but went on to argue that Clinton accurately stated that jobs in the coal industry are dwindling. He also touted her proposals to help communities where coal jobs are being lost.

Additionally, the former governor has celebrated an endorsement from the League of Conservation Voters Action Fund, the political arm of a green energy group that lobbies against coal-fired power plants.

"Only one candidate for Senate this November can be trusted to stand up for Ohio coal families and that is Rob Portman," Christian Palich, president of the Ohio Coal Association, told the Free Beacon in a statement. "The UMWA endorsement is well deserved since Senator Portman has been a steadfast friend of miners, fighting everyday against radicals in Washington who seek to destroy their jobs."

The Ohio Democratic Party reacted to the endorsement in a statement, accusing Portman of "pushing the agenda of the wealthy and well-connected [that] has been devastating for Ohio’s coal workers."