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Republicans and Democrats have settled into the final weeks of a high-stakes fight for two years of Senate control. The party that wins a majority will not only dictate what policies the president can pass — on issues from health care to taxes and climate change — but also play a role in who he puts on federal courts.
The GOP holds a 53-47 advantage in the Senate. The party has to defend 23 seats this year, compared with 12 for Democrats. To gain control of the chamber, Democrats need to flip a net three seats if Biden wins (or four if he loses).
The Republican-held Senate has bolstered Trump during his first term in office. Not only has it confirmed a flood of his judicial appointments and cast aside a bevy of legislation passed by the Democratic-held House, but also it acquitted the president this year after the House impeached him.
Read below for a look at the states and candidates that will decide the battle for the Senate.
Democratic Sen. Doug Jones has the toughest path to reelection of any incumbent this year. He faces Republican former college football coach Tommy Tuberville in a state Trump won by nearly 30 percentage points in 2016.
Jones, 66, won a 2017 special election to complete Republican Jeff Sessions' term when the senator left to become Trump's attorney general. Tuberville, 66, then beat Sessions in this year's GOP primary.
Jones has touted bipartisan work as he represents a red state, including in trying to rein in the Trump administration's trade war with China. Still, he has sided with Democrats on some of the most important votes he has taken.
Jones voted to remove Trump from office after his impeachment trial and opposed Supreme Court Justice Brett Kavanaugh's confirmation. The GOP also targeted him in September for voting against a Republican coronavirus stimulus bill that Democrats unanimously opposed.
Tuberville, who coached at Auburn University in Alabama, has run as a staunch Trump ally. He has pledged to repeal the Affordable Care Act and push back on gun safety measures, among other policies.
Another one of the toughest races for senators running this year, the Arizona contest pits Republican Sen. Martha McSally against Democratic former astronaut Mark Kelly.
The special election will decide who serves the remainder of the late Sen. John McCain's term into 2023. McSally, 54, was appointed to the Senate in December 2018 after she lost the race for Arizona's second seat in the chamber to Democratic Sen. Kyrsten Sinema.
The state is one of a handful that will play a major role in determining both the Senate majority and the resident of the White House. Trump won the state by more than 3 percentage points in 2016, but Democrats see the opportunity for more gains there this year.
Kelly, 56, has consistently led in polls and raised more money than McSally. In another election year when health care is top of mind, Democrats have criticized the incumbent for her vote to repeal and replace the Affordable Care Act as a House member in 2017.
McSally, an Air Force veteran, has promoted her efforts to combat the coronavirus pandemic. She joined this year with Sens. Mitt Romney, R-Utah, and Susan Collins, R-Maine, to put forward an unemployment benefits plan more generous than the one her party's leaders initially proposed.
Colorado has recently become less hospitable for the GOP statewide. Hillary Clinton carried the state by about 5 percentage points in 2016. Then, Democratic Gov. Jared Polis won it by more than 10 percentage points in 2018.
Gardner, 46, has touted work with Democrats during his first Senate term on issues including the relaxation of marijuana laws and public lands preservation. But the opposing party argues he has not done enough to check the president. He was among several GOP senators running for reelection who voted not to remove Trump from office earlier this year.
Hickenlooper, like most Democrats seeking Senate seats this year, has run on preserving health-care coverage. The 68-year-old has criticized the chamber over Congress' delays in passing coronavirus relief.
While early polls found Hickenlooper leading the race, a ruling that he violated state ethics rules as governor sparked criticism from both his Democratic primary opponent and Republicans.
Democrats have not won a Senate or presidential race in Georgia since 2000. The party has ambitious goals for 2020 with two Senate seats and the state's electoral votes up for grabs.
Republican Sen. David Perdue, 70, seeks his second term in the Senate. He faces 33-year-old Democrat Jon Ossoff, an investigative journalist who lost a closely watched U.S. House special election in 2017.
Perdue, a Trump ally, has faced backlash for a flurry of stock trades he says an advisor made near the start of the pandemic, including investment in at least one company that makes personal protective equipment. He has denied wrongdoing. The senator has said his portfolio manager has since stopped investing in individual companies.
Ossoff has focused much of his campaign messaging on Republicans' handling of the pandemic. He went into isolation at one point during the race after his wife, Emory University physician Alisha Kramer, tested positive for Covid-19.
Perdue won his Senate race by about 8 percentage points in 2014. Polling averages have found him leading Ossoff.
GOP Sen. Johnny Isakson's retirement from the Senate last year opened up a messy race to succeed him. Sen. Kelly Loeffler, the 49-year-old Republican appointed to succeed him, faces a tough path to keeping the seat this year.
Polls show Loeffler, Republican U.S. Rep Doug Collins, Democratic pastor Raphael Warnock and Democratic teacher Matt Lieberman leading a packed field. If no candidate garners more than half of the vote – which appears likely – the race will go to a runoff between the top two candidates in January.
Loeffler faced criticism after she and her husband, Intercontinental Exchange CEO Jeffrey Sprecher, started a sale of more than $1 million in stocks on the same day as a senators-only briefing on the pandemic in January. Loeffler has denied wrongdoing, and the Justice Department eventually dropped a probe into the sales.
Still, opponents such as Collins have seized on the episode as evidence Loeffler is unfit for office.
Loeffler, owner of the WNBA's Atlanta Dream, has also criticized the basketball league for supporting the Black Lives Matter movement this year during a national reckoning over systemic racism. Players across the league then donned shirts supporting Warnock.
Lieberman, the son of former senator and vice presidential candidate Joe Lieberman, has faced calls to drop out because of a book he wrote in 2018 that the head of the state NAACP said contains "racist tropes." His presence in the contest could potentially hurt Warnock's efforts to secure a spot in a runoff election.
Trump ran away with Iowa in 2016. Four years later, polls suggest the state's presidential and Senate races could be competitive.
Republican Sen. Joni Ernst, 50, seeks a second term in the Senate. She faces 56-year-old Democratic businesswoman Theresa Greenfield in November.
Ernst won her first Senate election by more than 8 percentage points in 2014. While the senator enters the final stretch of the election with more cash at her disposal than Greenfield has, polls have found a close race for the seat.
Ernst has touted her work on economic issues in her first term, including pushing for the United States-Mexico-Canada Agreement and the year-round sale of gasoline blended with up to 15% ethanol. Both were major concerns for farmers in Iowa.
Greenfield has focused in particular on Ernst's vote to repeal the Affordable Care Act in 2017. She says rolling back the health-care law would jeopardize the state's Medicaid expansion and coverage for people with preexisting conditions.
Republican Sen. Susan Collins faces the toughest reelection bid of her more than 20-year Senate career. She runs against Democratic Maine House Speaker Sara Gideon, 48, in a state that typically leans blue in presidential election years.
Collins, 67, seeks her fifth term in the Senate. While the senator has long tried to build a record of bipartisan cooperation, her efforts to appease a broad swath of voters have proved more difficult during Trump's first term.
While she cast a critical 2017 vote against repealing the Affordable Care Act, Collins also sparked outrage among liberals the next year by voting to confirm Kavanaugh. The senator has most recently touted her role in crafting the Paycheck Protection Program loans that buoyed many small businesses during the pandemic.
Gideon, like most Democrats trying to unseat Republicans this year, has run a campaign based around preserving health-care coverage. Polling averages have found a narrow edge for the Democrat.
In 2018, Democratic Gov. Janet Mills and independent Sen. Angus King (who caucuses with Democrats) won statewide races in Maine. Democratic Rep. Jared Golden flipped the state's 2nd Congressional District in a narrow victory over Republican incumbent Bud Poliquin.
Clinton won the 2016 statewide vote by about 3 percentage points, and won three of Maine's four electoral votes (which it partly apportions by congressional district).
Democratic Sen. Gary Peters is one of only two senators from his party running in a state Trump won in 2016. The potential for Michigan to be competitive gives Republicans hope John James can flip the seat.
The president carried Michigan by a razor-thin margin of about 11,000 votes in 2016. Polling averages show Biden leading a close race there this year following a series of Democratic triumphs in 2018.
Peters, 61, seeks a second term in the Senate after he tried to define himself as a bipartisan pragmatist during his first six years. As the ranking member of the Senate Homeland Security and Governmental Affairs Committee, the senator has recently taken a leading role in oversight of the U.S. Postal Service after concerns that changes overseen by new Postmaster General Louis DeJoy led to mail slowdowns.
James, a 39-year-old Army veteran and businessman, runs for Senate for a second time after losing to Democratic Sen. Debbie Stabenow in 2018. James and Republicans have characterized Peters as a reliable vote for Democrats who has failed to make ground on local issues.
Polling averages have found a lead for Peters in a competitive contest.
Two-term Democratic Gov. Steve Bullock's entry into the Montana Senate race this year instantly made the contest more competitive. But he would still have to overcome a challenging environment for his party to unseat first-term Republican Sen. Steve Daines.
The last time Trump topped the ticket in Montana in 2016, the president carried the state by a whopping 20 percentage points. Even so, Bullock won reelection by 4 percentage points in the same year.
Daines, 58, has touted recent work on bipartisan public lands legislation. He has also promoted efforts to secure federal funds to rein in wildfires that burned in the state in early September.
Bullock, 54, has put particular emphasis on his health-care record as governor. The state expanded Medicaid during his tenure.
The rivals both entered the final months of the race with more than $7 million in the bank. Polling in the race has been limited relative to many other states, but surveys show an edge for Daines.
North Carolina could swing the races for both the White House and Senate majority. The state appears to pose bigger problems for the GOP this year than it did in 2016.
Trump carried the state by more than 3 percentage points as Republican Sen. Richard Burr won by an even wider margin. In 2020, polling averages show Trump locked in a dead heat with Biden, while they find GOP Sen. Thom Tillis trailing Democratic challenger Cal Cunningham in a close race.
Tillis, 60, unseated incumbent Democratic Sen. Kay Hagan in 2014. Tillis has recently tied himself closely to Trump on issues including the president's impeachment and the emergency declaration diverting military funds to construction of a border wall. He has run as a staunch supporter of lower taxes and the Paycheck Protection Program.
Cunningham, a 47-year-old former state senator, has pushed for the Senate to pass more coronavirus aid during a months-long impasse over providing more relief. He has also backed Medicaid expansion in North Carolina.
Democrats have high hopes for flipping a handful of U.S. House seats in Texas this year. Winning statewide will be another challenge entirely.
Republican Sen. John Cornyn, 68, seeks a fourth term in the Senate. Democratic Air Force veteran MJ Hegar, 44, hopes to upset Cornyn and notch her party's first U.S. Senate election win in Texas in more than 30 years.
When Trump last led the Republican ticket four years ago, he carried Texas by about 9 percentage points. During a strong midterm year for Democrats in 2018, Democratic former Rep. Beto O'Rourke fell about 2.5 percentage points short of GOP Sen. Ted Cruz.
Cornyn, like many of his GOP colleagues, has run on the funding his state got from coronavirus aid packages. He has touted legislation passed in recent years including a law to strengthen gun background checks and the 2017 GOP tax plan.
Hegar has targeted Cornyn both over his support for repealing the Affordable Care Act and his push this year for liability protections for businesses as part of coronavirus relief legislation.
Polling has found a significant lead for Cornyn.
— Graphics by CNBC's John Schoen.