Montana’s primary election election results Tuesday set up what is expected to be close matchup between incumbent Democratic Sen. John Tester and Republican newcomer Tim Sheehy.

The Montana Senate seat is one of 18 that Democrats have on the ballot this November as they fight to maintain control of the Senate. Republicans, meanwhile, are defending 11 seats in Senate races across the country. Independents hold five seats that are up for election.

Montana is one of the Cook Political Report’s three “toss-up” states, alongside Nevada and Ohio, that could decide which party controls the Senate.

Two out of the three “toss-up” states are in the West — not to mention Arizona, which will also have a competitive Senate race in November — which means voters in the West will play a crucial role in determining which party controls the Senate.

Here’s a closer look at key battleground states to watch in the 2024 Senate race:

‘Toss-up’ states

Montana

Incumbent Democratic Sen. Jon Tester will face off against Republican Tim Sheehy.

Sheehy, a Trump-backed Navy SEAL, won Montana’s primary election Tuesday, setting up what’s expected to be a contentious matchup.

Tester, who has narrowly survived three previous elections, is a Democrat in a state that overwhelmingly voted for Trump twice, making him “the most vulnerable senator running for reelection,” according to CNN’s assessment.

A loss by Tester would not only mean a flipped seat in favor of the GOP, but would also mark the removal of the only Democrat currently holding a high office in the state.

Ohio

A March 19 primary set a matchup between incumbent Democratic Sen. Sherrod Brown and Republican businessman Bernie Moreno.

Brown has a lot in common with Tester in Montana: They are both three-term incumbents and are the only two Senate Democrats running for reelection in Trump states. This means the seat has a good chance of flipping in a state that backed Trump twice.

Nevada

As Nevada’s primary election looms, Army veteran Sam Brown is the Republican favorite to go up against incumbent Democratic Sen. Jacky Rosen.

Both Brown and Jeffrey Gunter, Trump’s former ambassador to Iceland, have sought the former president’s endorsement in this key battleground state.

Nevada’s primary election will be held on June 11.

Other battleground states to watch

Arizona

A Senate seat is up for grabs in Arizona after independent Sen. Kyrsten Sinema announced in March that she would not be seeking reelection.

Democratic Rep. Ruben Gallego will likely face off against either former Arizona gubernatorial candidate Kari Lake or Pinal County Sheriff Mark Lamb.

Polls show Lake as the the presumptive GOP primary winner — setting up a key battle between Gallego and Lake in a race that could decide which party controls the Senate.

Arizona’s primary election will be held on July 30.

Maryland

Maryland, like Arizona, also has an open Senate seat going into this year’s election following Democratic Sen. Ben Cardin’s retirement.

A May 14 primary set up a contest between former Republican Gov. Larry Hogan and Democrat Angela Alsobrooks, a county executive.

Polling forecasts an unexpectedly close race in the deep blue state.

Michigan

Michigan also has an open Senate seat and even though the state has not elected a Republican to the Senate in 30 years, Cook Report finds that Democrats only have a slight lead.

U.S. Rep. Elissa Slotkin leads in the polls on the Democratic side, while the Trump-backed Republican Rep. Mike Rogers leads on the Republican side of the primary race.

Michigan’s primary election will be held on Aug. 6.

Pennsylvania

Democratic Sen. Bob Casey will face off against Republican businessman David McCormick, as determined by an April 23 primary election.

Casey is running for a fourth term in what is expected to be a tight race.

Democrats only have a slight edge in the Senate race in Pennsylvania, Cook Report finds.

West Virginia

Democrat-turned-independent Sen. Joe Manchin’s open seat in Virginia, following his retirement announcement in November, gave Republicans in the red state a “clear path” to picking up the seat, The Associated Press reported.

A May 14 primary showed the Trump-endorsed Gov. Jim Justice was the strong favorite to compete for the open seat against Democrat Wheeling Mayor Glenn Elliott.

Wisconsin

Incumbent Democratic Sen. Tammy Baldwin will likely be facing off against the Trump-endorsed Eric Hovde, the Republican favorite to win the Wisconsin primary.

The race is projected to a be a close one, with the Cook Report rating it as leaning Democratic.

Wisconsin is the only state that Trump lost in 2020 where a GOP Senate candidate won in 2022, per Forbes.

Wisconsin’s primary election will be held on Aug. 13.