U.S. Senator Jon Tester (D-MT) attends a Senate Democratic caucus meeting on Capitol Hill in Washington, U.S., September 28, 2023. / Reuters/Craig Hudson

WASHINGTON (Reuters) -Democrats face a tough fight in the Nov. 5 election to preserve their narrow 51-49 majority in the U.S. Senate, as they are defending several incumbents in Republican-leaning states.

Below are the key Senate races to watch:

MONTANA: VULNERABLE DEMOCRATIC INCUMBENT

Democrat Jon Tester's bid for reelection to a fourth six-year term in heavily Republican Montana will be one of the chamber's most competitive, according to a Reuters analysis of the three main nonpartisan U.S. election ratings services.

Tester, 68, has served since 2007 in the Senate, where he has portrayed himself as more independent, opposing energy regulations put forward by Democratic President Joe Biden and pushing the president to take additional action to strengthen border security.

The third-generation working farmer has never won a race with more than 51 percent of the vote.

Tim Sheehy, a 38-year-old former Navy SEAL who founded an aerial firefighting company and now manages several luxury real estate properties, has the backing of Republican former President Donald Trump.

OHIO: BROWN PLAYS DEFENSE

Democrat Sherrod Brown, 71, faces an equally competitive reelection campaign in a state where Trump has a great deal of support. Brown has won election in this increasingly Republican-leaning state three times since 2006, with each race closer than the last.

Like Tester, he has worked to develop a more independent reputation than many of his fellow Democrats, including pushing bills to protect American manufacturing jobs and holding financial institutions accountable from his perch as chair of the Senate Banking Committee.

His opponent, Bernie Moreno, 57, owned a group of car dealerships. Moreno’s family emigrated from Colombia when he was a child, and he has emphasized a childhood of humble origins in the U.S., although his family is well-connected to Colombian elite.

MICHIGAN: COMPETITIVE RACE FOR OPEN SEAT

With Democratic Senator Debbie Stabenow retiring, Michigan's Senate race is also forecast to be highly competitive in a Midwestern state which will also play a pivotal role in the presidential election. Anger among Arab American voters in the state over the Biden administration's handling of the war in Gaza could pose a problem for Democrats.

Representative Elissa Slotkin, 48, a former CIA analyst who currently represents a swath of communities west of Detroit, easily won the Democratic nomination. Slotkin is a moderate who flipped a Republican-held House district in 2018, the same seat that Stabenow once held before her Senate run.

Republican former Representative Mike Rogers, 61, who headed the House of Representatives Intelligence Committee from 2011 to 2015, will run against Slotkin. In between his House career and Senate campaign, he was a lobbyist in Florida but returned to Michigan to run.

ARIZONA: SUCCEEDING SINEMA

Arizona, a battleground state in the presidential election, will also hold a competitive election to fill a Senate seat vacated by Democrat-turned-independent Senator Kyrsten Sinema, who is retiring.

Republican Kari Lake, 55, a former television journalist, first became known on the national stage when she ran for governor in 2022, narrowly losing to Democrat Katie Hobbs. This time she is hoping to expand the appeal of Trump's heavily populist message among Arizona residents who have been hit hard by skyrocketing housing costs.

Democratic U.S. Representative Ruben Gallego, a 44-year-old former Marine who served in Afghanistan, is running on a platform that focuses on gun control and immigration, both hot topics in the border state.

PENNSYLVANIA: CASEY VS. MCCORMICK

Democrat Bob Casey Jr., 64, is seeking a fourth six-year term. The moderate senator is a popular figure in his home state and the son of a former governor.

Challenging him is Dave McCormick, 59, a former hedge fund executive who lost the Republican primary for the Senate seat in 2022.

The state is expected to be one of the most competitive at the presidential level as well.  

NEVADA: ROSEN FIGHTS TO STAY ON

Democratic incumbent Jacky Rosen, 67, is running for a second Senate term. The seat is a top priority for Republicans to flip.

The state - with an economy that relies heavily on tourism - is still struggling to recover from the economic impacts of the pandemic, and has experienced high cost-of-living and housing cost increases.

Nevada also has a large Latino population, a traditionally Democratic demographic where Trump and Republicans have been making significant inroads.

The Republican challenger is Sam Brown, 40, a military veteran who received a Purple Heart after being wounded by a roadside bomb in Afghanistan.

WISCONSIN: REPUBLICAN HOVDE CHALLENGES BALDWIN

Democrat Tammy Baldwin, 62, is seeking a third term in a state that is expected to be competitive. She will face Eric Hovde, a Republican, in the November general election.

Like Tester and Brown, Baldwin has supported more economically populist bills and helped pass a bill legalizing same-sex marriage at the federal level last year. She was the first openly LGBT woman elected to both the House and Senate.

Hovde, 60, is the chief executive of his family's real estate development corporation and also CEO of several banking companies he founded. He has drawn scrutiny for remarks questioning the voting capabilities of nursing home residents.

NEBRASKA: SLEEPER RACE TURNS COMPETITIVE

Republican Senator Deb Fischer, 73, was expected to win reelection handily in Nebraska, where Trump won with roughly 58 percent of the vote in both 2016 and 2020.

But polls have shown independent Dan Osborn, a union leader who refused the Nebraska Democratic Party's endorsement, closing in on Fischer. He is running on a populist platform that pulls elements from both parties: support for abortion rights and gun rights, slamming the power of corporate America, and describing illegal immigration as a "pool of cheap labor."

Osborn, 49, has said he would not caucus with either party, unlike the Senate's two current independent senators, who both caucus with the Democrats. If he wins, that could complicate control of the Senate, making this race one to watch.

NEW JERSEY: LOOKING TO REPLACE MENENDEZ

Democratic U.S. Representative Andy Kim, 42, is vying to succeed Senator Bob Menendez, who is resigning after being found guilty of federal corruption charges.

Kim, a former diplomat who has represented a New Jersey district since 2019, is a progressive campaigning on a family-centered platform, speaking often about his two young boys.

Curtis Bashaw, a 63-year-old hotelier who beat Trump’s pick in the Republican primary, is running against Kim.

Democrats are strongly favored to hold the seat in November, according to the Reuters analysis of nonpartisan rating services.

WEST VIRGINIA: NEAR-CERTAIN BET FOR REPUBLICANS

West Virginia is considered a near-certain pickup for Republicans, as Joe Manchin, a conservative Democrat turned independent, is retiring.

Governor Jim Justice, 73, is the Republican candidate. First elected to office as a Democrat, he changed party affiliation seven months later and won reelection as a Republican. He is known for bringing his English bulldog, Babydog Justice, with him on speaking engagements and ran a campaign encouraging people to get vaccinated during COVID-19 with the slogan "Do it for Babydog."

Glenn Elliott, the 52-year-old Democratic mayor of Wheeling, West Virginia, is running against him.

Justice is strongly favored to hold the seat in November, according to the Reuters analysis of nonpartisan rating services.