Mr. Justice said he would officially change his voter registration on Friday. “I will tell you with lots of prayers and lots of thinking, today I will tell you as West Virginians that I can’t help you anymore being a Democrat governor,” he said. He imagined his mother, who has passed away, saying, “Jimmy, it’s about damn time you came to your senses.”
The governor initially received some boos from members of the crowd who seemed uncertain why he was there. But they applauded his announcement, and he quickly wrapped his arms around the president. “This man is a good man. He’s got a backbone, he’s got real ideas. He cares about us in West Virginia,” Mr. Justice said of Mr. Trump. He defended the president over the Russia investigation, saying, “Have we not heard enough about the Russians?”
After the announcement, Mr. Trump praised the governor: “Having Big Jim as a Republican is such an honor,” he said. “Fantastic man, fantastic guy.”
Mr. Justice, a billionaire coal and real estate magnate, ran as a conservative Democrat and declined to endorse Hillary Clinton in 2016. But even as West Virginia has become a reliable Republican state in presidential elections and further down the ballot, a handful of Democrats have still been able to win office.
Mr. Justice, though, is said to have friendly relationships with some members of Mr. Trump’s family. Before entering politics he gave money to officeholders in both parties while variously registering as a Republican, a Democrat and an independent.
Mr. Justice’s decision to change parties will further isolate Senator Joe Manchin III, a West Virginia Democrat and himself a former governor.
Mr. Manchin has resisted the entreaties of Republicans to change parties and add to the two-seat Senate majority. Some Trump administration officials, eyeing Mr. Manchin’s seat, also had hoped to put him in the cabinet, but he has declined all overtures. Now he must stand for re-election in 2018 without the help of a friendly governor.
“Joe Manchin has been and always will be a proud West Virginia Democrat,” said Jonathan Kott, Mr. Manchin’s communications director. Mr. Kott said Mr. Manchin was still not interested in joining the Trump cabinet.
In his first months as governor, Mr. Justice, a colorful and hulking figure who stands over six feet, seven inches, drew headlines beyond West Virginia’s border for bringing a plate of cow excrement into the state Capitol to express his anger at a Republican-passed budget proposal.
“What we have is nothing more than bunch of political bull you-know-what,” Mr. Justice said, pulling the lid off a silver platter to reveal the scented prop atop a printed copy of the budget. But the governor, who found his fortune with coal and now owns The Greenbrier resort, continues to be dogged by the financial issues that Republicans sought to use against him in last year’s campaign.
The state tax department has filed four liens this year against of one of Mr. Justice’s companies, Tams Management Inc., citing nearly $1 million in unpaid taxes, interest and penalties related to his coal mines.
Mr. Justice scaled back his television advertising campaign late in last year’s governor’s campaign, telling Democratic officials that, while wealthy on paper, he had limited liquidity, according to a Democrat involved in the conversations.
Representative Evan Jenkins, one of the Republicans seeking Mr. Manchin’s Senate seat, said he would welcome the governor to his party.
“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,” said Mr. Jenkins, who switched parties several years ago.
Shaking hands with constituents earlier in the day at a meet-and-greet near the stadium where Mr. Trump was scheduled to speak, Mr. Jenkins said he would understand if Mr. Justice felt the same way.
“If Jim Justice is the next to change his party registration to Republican, I welcome him to the Republican party,” he said.
Chris Goertler, a retired consultant from Williamstown, W.V., who was at the stadium to see Mr. Trump, said he used to be a Republican but is now “nonaffiliated.” Told of Mr. Justice’s expected announcement, Mr. Goertler shrugged and said, “I could never be a Democrat myself.”
Russell Lucas, an electrician from Kenova, W.V., who was also in line waiting to see Mr. Trump, said he would welcome Mr. Justice to the Republican Party, but added that he supported him even though he was a Democrat.
“Doesn’t matter to me,” he said. “He’s done a lot for this state already. He’s a mine owner, and I know guys that work in the mines, and all of them like him. They said he’s for the people.”
Mr. Trump repeatedly reminded the crowd of his 42-point margin of victory over Mrs. Clinton in the state. “I love your grit, your spirit,” he said. “And I love our coal miners, and they are coming back strong. The change you voted for is happening every single day.”