West Virginia Republican Gov. Jim Justice announced Thursday that he is running for Democrat Joe Manchin’s U.S. Senate seat in 2024, setting up what could be a hard-fought campaign between two of the state’s most prominent politicians.

Justice, an ultrawealthy businessman who is barred from running for a third consecutive term as governor, ended months of speculation with the announcement at a posh resort he owns in White Sulphur Springs, West Virginia. It came on Justice’s 72nd birthday.

“I absolutely will promise you to God above that I will do the job, and I will do the job that will make you proud,” Justice told a crowd to a standing ovation.

Justice’s entry increases the spotlight on what will be one of the most watched 2024 races in the closely divided U.S. Senate, with the GOP hoping to flip the seat in the overwhelmingly Republican state. Manchin is one of three Democratic senators up for reelection in 2024 who represent a state won by former President Donald Trump in the 2020 election. He has been the only Democrat elected to statewide office in West Virginia since Justice switched his party affiliation in 2017.

Among the speakers at the announcement were West Virginia’s other U.S. senator, Republican Shelley Moore Capito, and South Carolina GOP Sen. Lindsey Graham.

Earlier Thursday, Justice’s campaign filed a statement of organization for a Senate run on the Federal Election Commission’s website.

“Jim Justice is a proven winner whose record of creating jobs, cutting taxes, and fighting for conservative values has made him one of the most popular governors in the country,” National Republican Senatorial Committee chairman Steve Daines said in a statement.

Manchin, who endorsed Justice in his first gubernatorial bid before Justice switched parties six years ago and their relationship deteriorated, has said he’ll decide his future political plans in December. Manchin didn’t wait for Justice to make his announcement to throw out a warning Thursday morning.

“I am laser focused on doing the job West Virginians elected me to do — lowering healthcare costs, protecting Social Security and Medicare, shoring up American energy security and getting our fiscal house in order,” Manchin said in a statement. “But make no mistake, I will win any race I enter.”

Likely to Be Manchin’s Toughest Election

Manchin’s federal campaign filings show he already has more than $9.7 million cash on hand. If Manchin seeks reelection, Justice’s announcement means Manchin is likely in for his toughest test in his three decades in West Virginia politics.

Justice criticized both the domestic and foreign policies of Democratic President Joe Biden, but did not mention Manchin at the announcement.

“This nation needs us as Republicans to win,” Justice said. “I’ll ask you to run right with me and absolutely, as we have so many times, run to the finish line together.”

The Senate bid by Justice sets up a matchup in the 2024 GOP primary with U.S. Rep. Alex Mooney. Mooney jumped into the Senate race less than two weeks after winning his fifth term in the House last November. State Attorney General Patrick Morrisey, who lost to Manchin in the 2018 general election, has announced he will run for governor next year.

Mooney’s campaign already has started attacking Justice’s record as governor in a 30-second political ad this week.

No Democrats have announced a challenge to Manchin. But Vermont Sen. Bernie Sanders, a progressive, has suggested he would support a primary opponent to the conservative Democrat, who was responsible for watering down much of Biden’s social spending agenda.

As governor, Justice took advantage of Trump’s popularity amid a red wave in West Virginia. Justice was first elected as a Democrat, then switched to the GOP seven months after taking office, taking the stage at an event with Trump to make the announcement in 2017.

Justice said he told Manchin about the party switch the day before his announcement, keeping it secret even from the governor’s own staff. Manchin said the switch disappointed him.

Now foes, the pair have traded jabs ever since. In 2019, Justice in a news release called Manchin “one of those loud Washington liberals” who hadn’t embraced Trump. He blamed Manchin, a former two-term governor, for the state’s neglected road system. Manchin returned fire, saying “knowing Jim Justice’s character, it’s not a surprise for him to make a comment like this. He blames others for the work he hasn’t done.”

Justice is known for his exuberant and sometimes oddball method of delivery. He attends major events with his beloved English bulldog Babydog in tow, and Thursday’s announcement was no different.

In January 2023, Justice reintroduced an often-made proposal to cut the state’s personal income tax. Justice said legislative leaders told him to either give up trying or “make a big splash.”

“And so here comes me cannonballing into the pool,” the governor said at the time. “By God, I’ll make a big splash.”

After lawmakers agreed to a compromise tax cut of 21.25%, Justice signed it in early March.

Before going into politics, Justice bought the 6,500-acre (2,630-hectare) Greenbrier resort in White Sulphur Springs out of bankruptcy for $20.1 million. Within a year he constructed a bunker casino beneath the resort’s 721-room hotel and established a PGA Tour golf event that was held on the Old White TPC for 10 years.

Justice reached billionaire status through family-owned farms and coal mines. His worth peaked at $1.7 billion, but he was taken off Forbes’ prestigious list of billionaires in 2021.

Justice’s businesses faced several other woes, including millions of dollars in penalties from the federal government and court fights over claims his companies failed to deliver coal, pay tax debts owed to counties or contribute toward monthly premiums on employee benefit plans.

During his first years as governor, he didn’t spend a lot of time at the seat of government in Charleston, instead choosing to commute from his home in Lewisburg, West Virginia. That prompted a lawsuit by a lawmaker who called him a “part-time governor.” The case concluded in 2021 when Justice agreed to live in Charleston.