The Senate battlefield is increasingly locked in as Republicans turn their attention from the primaries to the November contests in a bid to turn the chamber red.

Much has changed in the four months since The Hill last updated this space, including retirements, surprise candidacies and primary wins.

Republicans have settled on nominees in key states as they look to retake the upper chamber, while Democrats are looking at their crop of battle-tested incumbents to carry the year for them. And both sides are laser-focused on Ohio and Montana as the two most competitive contests.

At the same time, there are warning signs flashing at the top of the ticket, as President Biden has been unable to reverse his polling drop, giving Republicans a shot in the arm more than seven months out from Election Day.

Both sides readily admit West Virginia will fall into the GOP column in November, with Democratic Sen. Joe Manchin’s retirement from the deep red state. That means Republicans only need to flip one other state to control the upper chamber.

Here are the five other Senate seats most likely to flip.

Montana

Republicans are breathing easier after Rep. Matt Rosendale (R-Mont.) ended his Senate campaign, handing the primary to businessman Tim Sheehy and giving the GOP a clearer shot at defeating Sen. Jon Tester (D-Mont.).

Tester is a top target of Republicans, who are hopeful they can flip the deeply red state in November. And Sheehy was a top recruit of Senate Republican leaders.

Both campaigns have been on the air for months, with Tester holding a slight edge according to the latest public polling. An Emerson College/The Hill survey released in early March showed Tester with a 2-point advantage over Sheehy, who is steadily building name ID.

Tester also leads by 10 points among independents, while 14 percent of Trump supporters report that they back the three-term Democrat. 

But whether he can overcome the impact of Trump in the state remains a real question. 

The former president won the state by more than 15 points four years ago, with that margin potentially increasing as President Biden’s standing in the polls has dipped nationwide.

“It’s going to be tough,” one Democratic operative conceded. “But Montana has that ability to zag when people zig.”

Ohio

Republicans got the last piece of the GOP candidate jigsaw puzzle when businessman Bernie Moreno prevailed in a three-way contest for the right to take on Sen. Sherrod Brown (D-Ohio), who is looking to overcome the state’s increasingly red hue to secure a fourth term in office.

The final weeks saw the race narrow between Moreno and state Sen. Matt Dolan (R), but Trump world and undecided Republicans powered him to a 17-point victory.

For some national Democrats, Moreno, who was endorsed by Trump, is the perfect candidate for Brown to take on this fall — the Senate Majority PAC believed it to the tune of a $2.7 million ad campaign to boost Moreno ahead of the primary. 

“I think Democrats were right to want [Moreno]. If it was Dolan, there’s zero chance Brown returns,” one GOP operative said. “I think Moreno is the favorite today, but it’s not a lock by any stretch.”

Ohio has trended rightward for years, but Republicans aren’t casting doubt on Brown, who has proven to be a difficult opponent for years and is in a strong spot financially. 

“I would never underestimate Sherrod Brown,” former Rep. Steve Stivers (R-Ohio) said. “He’s the best retail politician in Ohio.”

The GOP operative specifically cited Brown’s recent ad featuring law enforcement officers touting Brown’s work on the fentanyl crisis.

“I would run that in a GOP primary,” the operative said.

Arizona

Sen. Kyrsten Sinema’s (I-Ariz.) decision not to seek a second term has removed a lot of the intrigue from the Grand Canyon State, clearing the path for what is expected to be a close race between Rep. Ruben Gallego (D-Ariz.) and Republican Kari Lake.

Sinema’s exit from the race initially boosted Lake’s chances, because the sitting senator was pulling GOP support at roughly double the rate of Democratic voters.

It also pushed a number of Senate Republicans, who had stayed on the sidelines out of deference to Sinema’s track record of bipartisan work, to back Lake. And it encouraged Republican operatives who are hopeful Sinema’s decision will open up some wallets in the business community.

Still, recent chatter hasn’t been as good for Lake, who was defeated in the 2022 gubernatorial election, as Republicans have grown increasingly bearish about her chances against Gallego.

An Emerson College poll conducted in mid-March showed Gallego receiving 44 percent support and Lake pulling in 40 percent.

Lake’s decision last week not to defend her past statements about an Arizona election official who is suing her for defamation added a layer of doubt.

“I don’t think we can ever be confident in Lake,” the GOP operative said. 

Republicans are also dismayed that Gallego is already up on the airwaves in an effort to define himself, and he is doing so unopposed. For a candidate largely considered progressive and not in the mold of Democrats who have won statewide in Arizona, Democrats believe he is doing everything he needs to.

“There’s a reason why they elected Sinema and [Sen. Mark Kelly (D-Ariz.)] is so popular,” the Democratic operative said, noting that Arizona Gov. Katie Hobbs (D) did not run a top-notch campaign against Lake last cycle, and still she pulled off a squeaker.

“She ran a meandering race, she didn’t handle the debate question well. There was a lot of angst pre-election,” the operative continued. “And she ended up winning.”

Pennsylvania

The air wars in the Keystone State started in earnest this week as Sen. Bob Casey (D-Pa.) and Republican David McCormick both went up with their first batch of television ads.

McCormick faces an uphill climb to oust the three-term incumbent despite his financial might, but national and state Republicans are pleased with the former Bridgewater Associates CEO’s campaign thus far.

Still, their fears that toppling Casey might be too tall of a task haven’t been fully assuaged.

The GOP operative contrasted McCormick’s route to victory for McCormick to Lake’s in Arizona. For Lake, the hope is to run as close to Trump as possible and pull off a narrow win in a state where margins of victory on either side are slim.

McCormick’s path is much different and would likely mirror what former Sen. Pat Toomey (R-Pa.) pulled off in 2016, when Trump outran him in deep red and rural counties, but he ran strong in the Philadelphia collar counties and earned a 1.5-percentage point win. 

To do that, McCormick has to either give Biden supporters a reason to back him, or rely on an unforced error by Casey, who isn’t known for them.

“He needs to get Biden/McCormick voters, and to do so you need an issue in which Casey has done something drastically wrong,” the operative said, noting that Toomey messaged heavily on the Iran nuclear deal that cycle to great success in southeastern Pennsylvania. “What is that thing to peel off Biden voters? Because there’s just drop-off from Trump voters to Dave, and there’s nothing they can do to fix that.”

“[McCormick’s campaign] is not doing it wrong. I just fear they can be perfect and still not be able to win,” the operative continued. 

According to the latest survey conducted by Emerson/The Hill, Casey leads McCormick by 4 percentage points. The incumbent Democrat holds that advantage despite Trump topping Biden by the same margin.

Wisconsin 

The entrance of Eric Hovde, a wealthy businessman, into the Wisconsin Senate race is giving the GOP something it hasn’t had since Sen. Tammy Baldwin (D-Wis.) first won her seat 12 years ago: a real chance at defeating her.

According to the latest Emerson/The Hill survey, Baldwin holds a 3-point advantage over Hovde, who is expected to spend in the neighborhood of $20 million of his own money on the contest. Hovde went on the air almost immediately after he announced his campaign and has been running television ads since. 

Republicans are also heartened by some key numbers early on, including that Baldwin is short of 50 percent support in polls and that only 42 percent approve of her work, according to the latest Marquette Law School poll, taken in January. 

Democrats, though, remain confident that Baldwin’s winning ways will continue and that she is doing everything she needs to.

They also constantly point to their ongoing attempt to define Hovde as an out-of-touch carpetbagger who is trying to nab the seat and isn’t “one of us.”

“The contrast there is great,” said a Democratic strategist with experience working on Senate races, pointing to Baldwin’s recent Dairyland tour and emphasis on rural areas. “Tammy talking about rural priorities and going to places Dems don’t always go in Wisconsin compared to a guy who flew in from California to try to buy a Senate seat.”