On Thursday, the Democratic Party’s 2024 defense of the U.S. Senate will officially begin, as Governor Justice of West Virginia is set to throw his hat into the ring against Senator Manchin, which could make Mr. Manchin the last in a long line of Democrats to occupy his Senate seat.

According to an invitation obtained by Politico, Mr. Justice will be hosting a “special announcement” at the Greenbrier, a historic resort in West Virginia owned by one of Mr. Justice’s companies. It will also be Mr. Justice’s 72nd birthday.

By entering the race, Mr. Justice would be turning up the heat on Mr. Manchin. Handicappers and pollsters have suggested over the past year that the mining mogul turned governor would likely defeat Mr. Manchin in a head-to-head contest.

Providing that Mr. Justice follows through and enters the race, Mr. Manchin would be the last expected major candidate in the West Virginia Senate race who has not announced an intention to run. Some have speculated that Mr. Manchin’s re-election odds are daunting enough that he may choose to run for president instead, though his chances in that contest aren’t necessarily much better.

Polling conducted by National Public Affairs in late March found that Mr. Justice would be an immediate favorite in the GOP primary, and other polling has suggested he would likely win the general election.

The National Public Affairs survey found that Mr. Justice would enjoy a 20-point lead in a Republican primary and would carry 43 percent of the vote, with Attorney General Patrick Morrisey at 21 percent and Representative Alex Mooney at 10 percent.

A Republican poll from February, commissioned by the Senate Leadership Fund and conducted by the Tarrance Group, suggests that Mr. Justice would have an edge against Mr. Manchin, whose popularity plummeted in the deep red state after he voted for the Inflation Reduction Act, handing President Biden a win.

The survey found that in a head-to-head matchup, Mr. Justice would enjoy 52 percent support compared to Mr. Manchin’s 42 percent. While it’s not an insurmountable deficit at this point in the race, it does suggest Mr. Manchin would start his campaign at a disadvantage.

“The data from this study shows that, across every single metric, Governor Jim Justice is far and away the strongest Republican candidate in the U.S. Senate race in West Virginia, and the only tested potential candidate who currently leads Joe Manchin on a trial ballot test,” the Tarrance Group survey reads.

If Mr. Manchin chooses not to run or runs and loses, it could mean his seat in the Senate would be occupied by a Republican for the first time since Senator Revercomb left the office in 1959.

In 2024, Democrats will be fighting an unfavorable Senate map. With a one-seat majority, every seat, including Mr. Manchin’s, will be subject to a difficult battle.

If Democrats lose in West Virginia, they will need to win every other Senate election, including in states like Montana, Ohio, and Nevada, and retain the White House — with its vice presidential vote — in order to remain in control of the upper chamber.

One hope for Mr. Manchin is the possibility that Republican infighting in the primary improves his general election prospects. In some ways the competition between Messrs. Justice and Mooney mirrors the national battle for influence in the GOP.

Although both have been historically friendly with President Trump, the influential Republican organization Club for Growth has thrown its weight behind Mr. Mooney, whereas Mr. Justice appears to have significant backing from Senate leadership.

Mr. Justice also has a potential vulnerability in the GOP primary because he ran for office as a Democrat, switching parties in 2017 to become a Republican and support Mr. Trump. Mr. Mooney has already taken to calling Mr. Justice a “RINO,” or “Republican in name only.”

Mr. Mooney has also touted his endorsements from West Virginia state legislators, asserting that “that should say something — that the people who’ve worked the closest with Jim Justice are endorsing me.”

Mr. Justice has responded by trying to paint Mr. Mooney as not being a true West Virginian, emphasizing his connection to Maryland.

“I mean, the truth of the matter is, Alex Mooney is from Maryland and absolutely, teetotally in every way connected to Maryland,” Mr. Justice said. Although Mr. Justice uses the word “teetotally” frequently, it’s not clear what it means. The Oxford English Dictionary cites it as an infrequently used reduplicated form of the adverb “totally.”