(NewsNation) — With the Democrats holding the slimmest of majorities in the U.S. Senate for the last two years — Vice President Kamala Harris playing tiebreaker on 50-50 votes — the balance of power once again could be upended during the 2024 election, with more than a dozen seats at play.

Polling in the months leading up to the general election has shown Republicans favored to win back control in the Senate, with Democrats defending their seats in some of the tougher-to-hold battleground states, including in Arizona, Florida, Michigan, Pennsylvania and Wisconsin.

For Democrats to keep their majority in the Senate, they’d need nearly all incumbents to win reelection and party members to win open seats in Arizona, Michigan and Maryland. Meanwhile, Republicans only need two seats in total to take the majority.

While Senate Democrats are feeling more optimistic about their chances down ballot with Harris at the top of the ticket, it remains an uphill battle and the party has poured more than $25 million into voter outreach in 10 states, according to the Democratic Senatorial Campaign Committee.

Here are the 13 Senate races worth watching this November:

Arizona: Kari Lake vs. Rep. Ruben Gallego

With independent Sen. Kyrsten Sinema not seeking reelection, voters in the battleground state of Arizona will be choosing between former President Donald Trump-backed Kari Lake and Democratic U.S. Rep. Ruben Gallego to replace her.

Arizona has been a critical battleground state in recent years, flipping blue in President Joe Biden’s favor in 2020 and in Gov. Katie Hobbs’ favor in 2022. However, Republicans continue to control both houses of the state legislature and a minimum majority in the state’s U.S. House delegation.

Lake is a former news anchor known for her staunch support of Trump and his false claims that he won the 2020 election. She lost her bid for Arizona governor in 2022 to then-Secretary of State Hobbs and made baseless claims that her election was also stolen.

Gallego, a lesser-known name nationally, is a Harvard-educated congressman who served in the Marines and has previously held office in Arizona’s House of Representatives, according to his campaign website.

The 2024 Arizona Senate race is considered a toss-up due to the Grand Canyon State’s evenly partisan lean, though Decision Desk HQ data currently forecasts a Democratic win.

Florida: Sen. Rick Scott vs. Debbie Mucarsel-Powell

Florida Democrats are making bold assertions about their chances to flip the historically conservative state, with some outlets reporting that it is among the 10 states where the Democratic Senatorial Campaign Committee has invested $25 million to expand voter outreach.

The state passed a “heartbeat” bill banning abortions after six weeks with few exceptions in 2023, and this year voters in the state will decide on a ballot measure to enshrine abortion access in the state constitution. As evidenced in 2022, abortion was a motivating issue for voters and could be again in Florida this year.

“Florida is in play,” proclaimed Democratic candidate for Senate Debbie Mucarsel-Powell at the start of a bus tour in defense of reproductive rights in Boynton Beach.

The former congresswoman from Miami is facing incumbent Republican Sen. Rick Scott in November. Scott has been the junior United States senator of the Sunshine State since 2019 following his tenure as the state’s governor from 2011 to 2019.

Recent polling has shown the two are neck and neck, even as Republicans have spent years cultivating their stronghold in the state and Gov. Ron DeSantis and Sen. Marco Rubio both won reelection by wide margins in 2022.

Despite Democrats’ optimistic framing ahead of the race, Decision Desk HQ polling forecasts a Republican win.

“Florida, long considered one of the core swing states, has trended towards Republicans in recent years and is now seen as a moderately red state at the federal level,” The Hill reports.

Maryland: Larry Hogan vs. Angela Alsobrooks

Historically blue, Maryland has the potential to swing Republican this November. Former GOP Gov. Larry Hogan threw his hat in the race for U.S. Senate, up against Prince George County executive and former state attorney Democrat Angela Alsobrooks. The two are gunning to take the seat of three-time Sen. Ben Cardin.

Recent polls have shown the race to be in a dead heat, though this week Alsobrooks appears to be favored by voters.

Hogan, who has continued to be very popular in a typically blue state, has sent Democrats scrambling this summer to reach voters in the state where many still don’t know the political outsider. Alsobrooks was even given a prime-time speaking slot at the Democratic National Convention last month in an effort to introduce her to voters on a national stage.

Decision Desk HQ forecasts show Alsobrooks has a more than 99% chance of winning the seat as of Sept. 16.

Michigan: Elissa Slotkin vs. Mike Rogers

Former Republican Rep. Mike Rogers — who notably disparaged Trump and then won his endorsement — faces Democratic U.S. Rep. Elissa Slotkin in the race to win the seat currently held by state’s senior senator and third-ranking Democrat in the chamber, Sen. Debbie Stabenow, who is retiring at the end of her term. It’s the first open Senate race for this seat since 1994, according to The Hill.

Slotkin, a fundraising powerhouse, jumped in the race soon after Stabenow’s announcement and immediately began raising funds for a battle in November. She faces a formidable opponent in Rogers, a former congressman who was lured out of retirement to go toe-to-toe against Slotkin.

Rogers even snagged a key endorsement in the state, the farm bureau known as AgriPac, which had supported Stabenow in her last two bids for reelection and hadn’t supported a Republican for U.S. Senate in nearly 20 years.

Decision Desk HQ suggests that Slotkin has an advantage, though the Cook Political Report classifies it as a toss-up.

Missouri: Sen. Josh Hawley vs. Lucas Kunce

Incumbent Republican Sen. Josh Hawley faces Marine veteran challenger Democrat Lucas Kunce in a race that has tightened in the months leading up to the general election in November, some polling has indicated.

Kunce, the director of national security at the American Economic Liberties Project, is attempting to unseat Hawley — who has held the seat since 2019.

After a lengthy dispute over whether or not the pair will debate, they finally settled on a Halloween night debate, just days before the election. The candidates accepted the invitation for the debate from NewsNation’s parent company, Nexstar Media Group, and will be streamed on NewsNation and other Nexstar stations across Missouri.

Current Decision Desk HQ projections put Hawley in the winner’s circle with a 93% chance of winning.

Montana: Sen. Jon Tester vs. Tim Sheehy

Sen. Jon Tester is looking to secure another Democratic win in his increasingly Republican state of Montana against Republican candidate Tim Sheehy, a decorated former Navy SEAL.

Tester previously won in 2006, 2012 and 2018, but winning a fourth term is harder to come by after Montana favored Trump in the 2020 presidential election, The Hill reports.

Sheehy, a businessman, has also received Trump’s endorsement.

DDHQ predicts a Republican win.

Nevada: Sen. Jacky Rosen vs. Sam Brown

Incumbent Democratic Sen. Jacky Rosen will face a newcomer in the GOP, Sam Brown.

According to Brown’s campaign website, he survived a roadside bomb explosion in Kandahar, Afghanistan, leaving him permanently scarred.

His political resume is scarce compared to Rosen’s multiple years holding Senate office, with DDHQ forecasts placing the Democratic candidate as the projected winner.

New Mexico: Nella Domenici vs. Sen. Martin Heinrich

Democratic Sen. Martin Heinrich, the incumbent who boasts two decades of political experience, is defending his seat against the GOP’s choice, Nella Domenici.

Domenici is the daughter of former U.S. senator and Land of Enchantment political icon Pete Domenici.

Like Missouri, the seat would be a hard one to flip. Heinrich is forecast to win with 93% of the vote, according to DDHQ forecasting.

Ohio: Sen. Sherrod Brown vs. Bernie Moreno

Democratic Sen. Sherrod Brown is looking to defend his seat in Republican-leaning Ohio, placing him in a similar position as Montana’s Jon Tester.

Brown, who serves as the chair of the Senate Banking Committee, will face off against businessman Bernie Moreno, who has garnered Trump’s favor and was not the Ohio GOP’s first choice for the race.

DDHQ gives Brown the edge, but Cook Political Report classifies the race as a toss-up.

Pennsylvania: Sen. Bob Casey vs. David McCormick

While Harris and Trump duke it out in the highly coveted Keystone State, longtime Democratic incumbent Sen. Bob Casey and GOP entrepreneur David McCormick will be squaring off for a Senate seat.

McCormick, the former chief executive of Bridgewater Associates — a massive hedge fund — previously ran against and lost to Mehmet Oz in the 2022 Republican Senate primary.

Casey has held public office in Pennsylvania since the late ’90s, and DDHQ puts him at a 77% chance of continuing his tenure.

Texas: Sen. Ted Cruz vs. Rep. Colin Allred

Democrats are attempting to dethrone Sen. Ted Cruz this November with another current Texas lawmaker: Rep. Colin Allred.

Allred, a former professional football player, shifted Cook Political Report’s rating of the Senate seat from “solid” Republican to “likely” Republican upon entering the race last year.

Cruz has faced off against Democrats and pulled off close victories before, like in his narrow 2018 win against Rep. Beto O’Rourke.

DDHQ and Cook Political Report both point to a Republican win for this race, but some aren’t sold.

“Anybody that says, ‘It just doesn’t happen in Texas’ — it doesn’t happen until it does,” pollster Brett Loyd told The Hill.

West Virginia: Jim Justice vs. Glenn Elliott

The retirement of longtime Sen. Joe Manchin III has paved the way for a Republican win in conservative stronghold West Virginia.

Since 2010, Manchin has been a rare Democratic leader in a deep-red state, with his exit from the race “setting the stage for one of the easiest flips for the Republican Party in recent memory,” The Hill wrote.

The state’s Republican governor, Jim Justice II, is poised to flip the state. His opponent, attorney and real estate broker Glenn Elliot, has a less than 1% chance of winning come November, according to DDHQ predictions.

Wisconsin: Tammy Baldwin vs. Eric Hovde  

Two-term incumbent Sen. Tammy Baldwin, a Democrat, is looking to defend her seat once again. In 2018, she won reelection by an 11-point margin. According to The Hill reporting, no Republican has won this Senate seat since 1952 — but that doesn’t mean there’s no chance for a GOP victory.

Her opponent is businessman and banker Eric Hovde.

According to The Hill reporting, no Republican has won this Senate seat since 1952, but that doesn’t mean there’s no chance for a GOP victory if the Badger State’s tight presidential election results — the last six were decided by less than 1% — have any say.

DDHQ predicts a Democratic win.

The Hill and The Associated Press contributed to this report.