West Virginia Gov. Jim Justice is running for Joe Manchin's pivotal Senate seat

West Virginia's Republican governor, Jim Justice, filed official paperwork to kick off a 2024 campaign for the U.S. Senate seat currently held by incumbent Democratic Sen. Joe Manchin.

A senior source close to the governor confirmed that Justice's campaign filed with the Federal Election Commission on Thursday.

The long-anticipated move by Justice, who switched parties in 2017 and has since become one of the red state's most popular politicians, could further imperil Democrats' efforts to keep their slim majority in the Senate.

Manchin, one of the chamber's most conservative Democrats, has said he won't decide whether to run for reelection until closer to the end of the year, regardless of Justice's plans. His seat is considered one of the most winnable for Republicans in next year's election.

"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 Thursday morning following news of Justice's Senate bid.

"But make no mistake, I will win any race I enter," Manchin added.

If Manchin decides not to run for a third term, Democrats are widely expected to lose the seat to whichever Republican nominee emerges from West Virginia's Senate primary. Rep. Alex Mooney, R-W.V. who was backed by former President Donald Trump in his most recent House election, has already jumped into the Senate race.

Mooney's campaign manager said Monday that it would be a "terrible idea" for Justice to run for the Senate, calling the governor a "Democrat in sheep's clothing."

Justice stood alongside Trump when he announced that he would switch to the Republican Party. A Trump campaign spokesman did not immediately respond to a request for comment.

Victory is hardly a guarantee for Manchin if he does try to hold onto the seat. Despite his incumbent advantage, his hefty war chest and his reputation for breaking with fellow Democrats, Manchin's party affiliation may too great an obstacle to overcome in a state that voted for Trump by nearly 40 points in 2020.

Manchin won his last Senate race in 2018 by just three percentage points. With his seat under threat, Manchin has appeared increasingly critical of President Joe Biden and his fellow Democrats.

The governor, who is term limited as governor, had been strongly encouraged to run for Senate by top Republicans, including NRSC Chair Sen. Steve Daines of Montana and Senate Minority Leader Mitch McConnell of Kentucky.

Justice, reportedly his state's wealthiest resident, is set to announce his campaign at 5 p.m. ET at The Greenbrier, his luxury hotel in White Sulphur Springs, West Virginia.

This is breaking news. Please check back for updates.