West Virginia Gov. Jim Justice is set to announce Thursday night that he is switching his party affiliation from Democrat to Republican at President Trump's rally in Huntington, W.V.

The news comes hours after President Trump told reporters at the White House that there would be a "big announcement" at the rally. The New York Times was first to report the planned announcement.

Justice took over the governorship in January after winning by 42 points last November over his Republican opponent, having declined to endorse 2016 Democratic presidential nominee Hillary Clinton prior to election day.

One GOP operative in the state said the change "really picked up steam" in the past two weeks and that Trump was the catalyst behind the move, particularly when the two traveled together to the National Boy Scout Jamboree on June 24.

"Virtually no one, with any certainty, knew this was coming," said the operative.

A former Republican and independent, Justice is a billionaire coal and real estate tycoon and was one of the few remaining Democratic statewide office holders. Among those are Sen. Joe Manchin, D-W.V., who is up for re-election in 2018 and said in a statement Thursday that he plans to work together with Trump on tax reform.

"The President is coming to West Virginia, and I'm excited he is. I know some Democrats, especially those in Washington, D.C., would not want the President to visit their state," Manchin said in the statement. "As Americans, we should always be honored to have a President visit our state whoever he or she is regardless of political party. All Americans should hope for the President to succeed, because if he does, the country succeeds and West Virginia does as well.

"I am particularly excited to work with him on tax reform," Manchin continued. "Our existing tax code is too complicated, burdensome, and hinders our economic development. Working West Virginians deserve a break and businesses need a level-playing field. Also, we can't continue to pile on debt for our grandchildren and future generations. If we work together and reject Washington partisanship, I think we can get that done."

Trump's rally in Huntington is expected to attract thousands of supporters from around West Virginia, where he ran up a huge victory over Clinton last November. Buoyed by his support of the coal industry, Trump won by a 42-point margin over his Democratic rival.

With Justice's announcement, the number of Republican governors will rise to 34, while the number of Democratic ones falls to only 15. Alaska's Bill Walker is the lone independent governor.