The 2020 elections are set to flip the script in the battle for the Senate, as Republicans must defend at least 20 seats, teeing up a series of potentially competitive races that could alter the political map.

Republicans have so far expanded their Senate majority going into 2019 to 52 seats. They could pick up one more if Sen. Cindy Hyde-Smith (R-Miss.) emerges victorious in her runoff election against Democrat Mike Espy, a former congressman and agriculture secretary.

But Democrats have already put a number of Senate seats in their sights as they look to recapture a majority in the chamber in 2020. Meanwhile, Republicans are eager to win back the Alabama seat currently held by Sen. Doug Jones (D).

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Here are five Senate races that are shaping up to be among the most competitive in 2020.

Alabama

Jones eked out a win in his 2017 special election bid against a Republican candidate shrouded in scandal and controversy. But as he prepares to run for his first full term in 2020, it’s unclear if he’ll be able to hold on in deep-red Alabama.

Alabama is the only solidly Republican state where a Democrat will face reelection in 2020, and while Jones doesn’t yet have a top-tier challenger, the GOP is already eyeing his seat as its best pick-up opportunity in the next election.

What’s more, Republicans are likely to go after Jones on his record in the Senate thus far, such as his vote against the confirmation of Supreme Court Justice Brett Kavanaugh Brett Michael KavanaughTrump faces tricky choice on Supreme Court pick The Hill's 12:30 Report: Trump stokes fears over November election outcome The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' MORE.

The Senate Leadership Fund, a super PAC aligned with Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellTrump 'no longer angry' at Romney because of Supreme Court stance On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline The Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power MORE (R-Ky.), hit the Alabama Democrat in a press release last week over his vote to reelect Sen. Charles Schumer Chuck SchumerPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' 3 reasons why Biden is misreading the politics of court packing Cruz blocks amended resolution honoring Ginsburg over language about her dying wish MORE (D-N.Y.) as minority leader.

Maine

Democrats are bullish about their chances of ousting Sen. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Billionaire who donated to Trump in 2016 donates to Biden Credit union group to spend million on Senate, House races MORE (R) in 2020.

Democrat Janet Mills easily won her bid to succeed retiring Republican Gov. Paul LePage this year, leaving Collins as the last remaining GOP statewide elected official. And with the defeat of Rep. Bruce Poliquin Bruce Lee PoliquinBallot measures across US aim to overhaul voting practices Dale Crafts wins Maine GOP primary to face Rep. Jared Golden House Democrats make initial ad buys in battleground states MORE (R-Maine) in Maine’s 2nd District, both of the state’s House members will be Democrats.

What’s more, the state has gone blue in every presidential election since 1988.

Collins incensed liberals last month when she voted to confirm Kavanaugh to the Supreme Court, prompting warnings of an aggressive push to boot her from office. Already, one prominent Democrat, former national security adviser Susan Rice, has floated the idea of challenging the four-term senator in 2020.

But Collins has a history of winning outsize reelection victories. In 2008, a bad year for Republicans, she crushed former Rep. Tom Allen (D) by a whopping 23 points. She won reelection once again in 2014 by a 37-point margin.

Arizona

Republicans have held both of Arizona’s Senate seats for well over two decades. But as evidenced by Democrat Kyrsten Sinema’s victory in the race to succeed retiring Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R) this month, the state’s hue is becoming more purple.

The 2020 Senate election in Arizona is set to determine who will fill the seat once occupied by the late Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R), a political giant who passed away earlier this year from brain cancer.

Arizona Gov. Doug Ducey (R) appointed Jon Kyl, a former Republican senator, to fill the seat after McCain’s death. But Kyl has said he does not intend to seek election in 2020, setting up the possibility of a highly competitive primary and making the seat a prime target for Democrats.

But Republicans are unlikely to cede the state without an aggressive fight. Kyl is considering leaving the Senate seat ahead of 2020, and speculation has swirled that Ducey could name outgoing Rep. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R), who lost her Senate bid to Sinema, as Kyl’s replacement.

If that happens, it could give McSally a second shot at the Senate seat and give Republicans an experienced candidate with an already high profile.

Colorado

Colorado has long been considered among the nation’s perennial battleground states. But that reputation appears to be waning, setting up a potentially tough reelection environment for Sen. Cory Gardner Cory Scott GardnerBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cook Political Report shifts Colorado Senate race toward Democrat MORE (R) in 2020.

Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE beat Trump in Colorado by roughly 5 points in 2016. And earlier this month, Democrat Jason CrowJason CrowClark rolls out endorsements in assistant Speaker race Trump-Afghan deal passes key deadline, but peace elusive Cook shifts 20 House districts toward Democrats MORE ousted Rep. Mike Coffman Michael (Mike) Howard CoffmanColorado mayor says he called protesters 'domestic terrorists' out of 'frustration' Colorado governor directs officials to reexamine death of Elijah McClain in police custody Petition demanding justice for Elijah McClain surpasses 2 million signatures MORE (R).

In the race for governor, Democrat Jared Polis Jared Schutz PolisMore than 1,000 gather at Colorado racetrack to protest governor's coronavirus orders Over 300 LGBT leaders endorse Biden for president : 'The most pro-equality ticket in US history' Colorado GOP lawmaker, Michelle Malkin sue governor over coronavirus orders MORE notched a comfortable win over Republican Walker Stapleton. Perhaps more notable is that the 2018 midterms gave Democrats control of every statewide office in Colorado, as well as majorities in both the state House and Senate.

All told, that sets the stage for a potentially fierce Senate battle in 2020. Gardner, chairman of the National Republican Senatorial Committee, has proven to be an adept campaigner. But his Senate seat is high on Democrats’ wish list as they seek to flip the last remaining Republican-held statewide office in their favor.

Iowa

Iowa may have handed Trump a nearly 10-point win in 2016, but the Hawkeye State holds a reputation as a perennial battleground, setting the stage for a fiercely competitive Senate race in 2020 when Sen. Joni Ernst Joni Kay ErnstTillis appears to reinforce question about COVID-19 death toll The power of incumbency: How Trump is using the Oval Office to win reelection Poll: Trump opens up 6-point lead over Biden in Iowa MORE (R-Iowa) is expected to seek a second term.

Democrats managed to flip two of Iowa’s Republican-held House seats earlier this month, meaning that the state’s congressional delegation will be evenly split between the two parties next year.

Former President Obama also carried the state in his 2008 and 2012 White House bids.

But there’s a silver lining for the GOP. The state’s Republican Gov. Kim Reynolds won a second term this month and her party held their majorities in the state House and Senate, maintaining the GOP’s trifecta in Des Moines.

Just missing the cut: North Carolina

North Carolina’s political future is in flux. The state has seen a surge in new residents in recent years, particularly in suburban areas and cities, such as Charlotte, that could create a more Democratic-friendly electorate.

At the same time, Democrats chipped away at the GOP’s supermajority in the state General Assembly this month. Still, Democrats didn’t see the same kind of success in U.S. House races in the state in 2018.