Sen. Joe Manchin’s (D-W.Va.) decision to retire has made the Senate landscape in 2024 even more complicated for Democrats.

The party was already facing an uphill slog to hold on to its Senate majority, as it prepared to protect seats in West Virginia, Montana and Ohio — three states where the GOP nominee for president will be heavily favored.

Manchin’s name on the ballot suggested a competitive race. Without him, the Senate seat seems wrapped for the GOP as a gift.

Losing the state would leave no room for another loss for Democrats if they hope to keep their majority, which now stands at 51-49 with Independent Sen. Kyrsten Sinema (Ariz.) voting with Democrats.

Still, the party is hopeful it can carry the day even as a recent run of polls show former President Trump, the frontrunner for the GOP presidential nomination, leading President Biden.

Trump’s rise has given a major boost to Republicans, who believe they can win not only in red territory, but purple states as well if that stays true a year from now.

Here are the five Senate seats most likely to flip.

West Virginia 

If gambling sites took political bets, they would have taken this race completely off the board this month after Manchin announced his retirement from the Senate.

The decision by Manchin — who was considered by most, if not all, to be the only Democrat with a prayer to keep the seat blue — virtually guaranteed West Virginia will go red next year and put Republicans one step closer to retaking the majority.

Manchin’s decision not to seek a third full term in office came as he was staring down a daunting match-up with West Virginia Gov. Jim Justice (R) and had seen his favorability ratings dip.

Justice is the heavy favorite to win the GOP primary over Rep. Alex Mooney (R-W.Va.), who is backed by the Club for Growth. Former President Trump last month threw his weight behind Justice, who is seemingly on a glidepath to replace the outgoing senator.

Manchin’s decision also does wonders for the GOP financially speaking.  

“There’s a real world where not only does Justice not have to spend any of his personal fortune, but that the GOP doesn’t have to spend any of ours either,” one GOP operative told The Hill. “That’s obviously huge, and it means more resources for the Pennsylvanias and Wisconsins of the world.” 

Montana

With Manchin out of the way, the battle for the Senate majority truly boils down to two states, and Republicans are desperate to oust Sen. Jon Tester (D-Mont.) and put Montana in their own column.

Like Manchin, Tester is widely considered the lone Democrat with a chance in Montana, a state Trump won by 16 points. But he, too, has a mountain to climb.

He is expected to square off against Tim Sheehy, a highly sought GOP recruit, in a presidential election year expected to feature Trump atop the ballot.

Tester isn’t a stranger to outperforming presidential nominees, having won by nearly 4 percentage points in 2012 while then-President Obama lost the state by nearly 14 points. But times have changed, and Trump remains a dominant figure in ruby-red territory such as Montana. 

Despite their confidence in Tester, Democrats are clear-eyed about the challenge ahead.

“This is going to be a hard, hard, hard race,” said John LaBombard, a former top aide to Sen. Kyrsten Sinema (I-Ariz.) and former Sen. Claire McCaskill (D-Mo.), and a Democratic strategist with ROKK Solutions. “But I won’t bet against Sen. Tester.” 

Tester is already up across the state with TV ads — a necessity, especially as Sheehy has been on the air since the summer as he seeks to introduce and define himself to voters. 

For Republicans, things seem to be moving Sheehy’s way. A pro-Sheehy Super PAC released a poll showing him leading by 3 percentage points over Rep. Matt Rosendale (R-Mont.), who has still yet to announce a bid for the upper chamber despite indications over the summer that he would. 

Ohio

Ohio Democrats haven’t seen many wins in the past decade, but that changed earlier this month when voters codified abortion rights into the state’s constitution. That gave Democrats a major shot in the arm as they look to follow suit next year and secure Sen. Sherrod Brown (D-Ohio) a fourth term. 

It won’t be easy. The political transformation of Ohio from the swingiest of swing states as recently as 2012 to reliable territory for Republicans is unmistakable.

Nevertheless, the Issue 1 vote that saw 56 percent of voters cement abortion rights is a blow to the state’s GOP — including a top Senate candidate.

Ohio Secretary of State Frank LaRose (R), one of the leading Senate candidates alongside businessman Bernie Moreno, has had a particularly difficult stretch. A GOP-led effort to raise the vote threshold needed for referendums failed earlier this year, and he had a lackluster third-quarter fundraising report that puts him far behind the eight ball. Moreno and state Sen. Matt Dolan (R) are both able to self-fund their operations. 

“On paper he’s impressive,” one Democratic strategist said of LaRose. “But how many of these candidates have we seen that are great on paper, but lack some of this X factor that the GOP base wants?”

LaRose and Moreno are considered the leading contenders for the primary nomination, but the main question at this point is whether Trump weighs in with an endorsement. The former president did just that two years ago and helped push Sen. JD Vance (R-Ohio) over the top in a six-way race. 

Moreno is considered the favorite to land it, but Trump has been much more measured in his Senate endorsements this cycle, having only done so for Justice, Kari Lake in Arizona and Rep. Jim Banks (R) in Indiana. 

No matter the candidate, the general election against Brown is shaping up to be one of the key races of the cycle as the incumbent looks to overcome what has become a reliably red electorate. 

Arizona

Republicans are making their first foray into the Arizona Senate race as they try to hit two birds with one stone while boosting GOP Senate candidate Kari Lake.

The National Republican Senatorial Committee rolled out a new ad with the goal of lifting incumbent independent Sen. Kyrsten Sinema among Democratic voters and taking a sledgehammer to Democratic candidate Rep. Ruben Gallego, all in a bid to raise Lake’s standing with Republicans.

The reason: Nearly a quarter of all GOP voters would back Sinema, according to a recent survey, prompting Republicans to attempt an early softening of that group. 

News of the effort was welcomed among Republican operatives, who are still questioning how real their chances are in the state as many jump balls remain in the air. Among those is whether Sinema sticks with her independent campaign or drops out entirely, and whether Lake continues to stick to the less-bombastic message she’s rolled out in the early stages of her campaign. 

“It’s good that they’re doing this now. It will tell us the long-term viability of this race before presidential spending is absurd in the state,” the GOP strategist said. “Thus far, it seems like Lake has learned some lessons, but that can change any day that she wakes up and does something off playbook.” 

The ad intensely attacked Gallego, labeling him a “deadbeat dad” without any evidence, and it took aim at him for a number of items related to his personal life, including his divorce. 

The poll released earlier this month showed him leading with 39 percent support, followed by Lake and Sinema with 33 and 29 percent backing, respectively. 

Pennsylvania

Sen. Bob Casey (D-Pa.) is likely facing his toughest reelection fight yet this year as he stares down a contest against David McCormick in one of the few match-ups that is virtually set in stone. 

McCormick has essentially cleared the field in a primary, giving him a free runway ahead of next November. That’s a far cry from what he encountered last year when he lost to Mehmet Oz in a razor-thin primary. 

He’s also starting to make some Democrats nervous, including Tester, who spoke frankly about the 2024 Senate lay of the land at a fundraiser recently.

“Bob Casey was supposed to have an easy race,” Tester told Democratic donors in Hollywood. “I’ve watched that race very closely, because it may not be near as easy as we were hoping.” 

Adding a layer to the Democratic nerves is Trump’s standing as he leads President Biden in a recent wave of polls. The ability for Trump to pull off a 2016 reprisal in Pennsylvania would also do wonders for McCormick, who will likely attempt to replicate former Sen. Pat Toomey’s (R-Pa.) victory in the same year. Toomey nabbed a second term by riding Trump’s coattails in the rural counties and peeling off enough votes in the suburbs. 

It’s not going to be easy, but the path is there, Republicans argue. 

“This is an uphill fight,” said Charlie Gerow, a Harrisburg-based GOP operative. “But it’s one that can and will be won.”