West Virginia's Democratic governor is planning to announce that he has switched to the Republican party at an event with Donald Trump, according to reports.

Sources told the New York Times that Governor Jim Justice will make his announcement at a West Virginia campaign rally with the President. The rally is expected to draw thousands of supporters from across the state, which Mr Trump won handily in last year’s election.

“We’re going to have a very big announcement, which will be very exciting I think for the media and everyone else,” Mr Trump told reporters ahead of the rally. “I believe I’ll be making it in West Virginia, so that will be very exciting.”

Mr Justice and Mr Trump share several things in common, starting with their business background. Mr Justice, a former coal and real estate magnate is a billionaire and is the wealthiest man in West Virginia. Just like Mr Trump, the Governor only assumed office this year, after winning the West Virginia election by 49 per cent.

Mr Justice, like Mr Trump, has wavered between the Republican and Democratic parties in the past. He previously donated to politicians in both parties, and has registered to vote as a Republican, Democrat and independent.

After being courted by both parties last year, Mr Justice eventually decided to run as a Democrat. The Democratic Governors Association spent more than $1m on his campaign, while the Republican Governors Association spent $3.7m against him.

On the day of Mr Justice’s expected announcement, the West Virginia Republican party was still attacking him on Twitter, writing: "Low-Energy @WVGovernor Refuses To Stop Millions Of $$ In Contracts To Companies Who Cheated WV Taxpayers. Sad!"

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Some West Virginia Republicans, however, said they would welcome the Governor to their party with open arms.

“I simply couldn’t be a part of a liberal agenda that was so contrary to who we are and what we believe in West Virginia,” Representative Evan Jenkins, who switched to the Republican party several years ago, told the Times.

The switch will not be too drastic for Mr Justice, who ran as a conservative Democrat and refused to endorse Hillary Clinton in 2016.

It will, however, be a shock for Senator Joe Manchin, the state’s lone, stubborn Democrat in Congress.