Since taking office in 2017, President Donald Trump has instituted dramatic policy changes that have hurt the most vulnerable U.S. residents — immigrants, children, women, transgender people, the poor. He's also made significant headway in transforming the federal judiciary into one that's whiter, more male, and less representative of the people it serves.

Trump has not acted alone, however: He's often been aided by the Republican-controlled Senate, led by Mitch McConnell of Kentucky.

As the Democrats organize to defeat Trump next year, they are also undertaking a major effort to win the Senate. There are 35 seats up in 2020, including special elections in Arizona and Georgia, and 23 of those seats are currently held by Republicans. That means Democrats must hold on to the 47 seats they currently control (two of them held by independents who caucus with the Democrats) and win at least three new ones — four if Trump is re-elected, since the vice president serves as a tie breaker in the 100-member body.

Races in a number of Southern states may prove crucial to making it happen.

Georgia, North Carolina, and Texas are among the states where Democrats are most likely to flip a Republican Senate seat in 2020, according to a recent Washington Post analysis. Among the non-Southern states where Democrats are most likely to capture Senate seats are Arizona, Colorado, and Maine. If each of those three non-Southern states end up flipping a seat, a single race in a Southern state could prove crucial to solidifying a Democratic majority in the Senate.

With that in mind, these are among the Southern states with Senate races to watch:

  • Alabama. Though typically viewed as a solidly red state, Alabama was the site of a special Senate election in 2017 in which Democrat Doug Jones defeated scandal-plagued Republican former state Supreme Court Chief Justice Roy Moore, showing what's possible when organizers spend time and energy engaging often-overlooked voters, like rural Black citizens. Jones is now running for his first full Senate term, one of only two incumbent Democratic U.S. senators facing re-election in a state Trump won in 2016. (The other is Gary Peters in Michigan.) There are currently seven Republicans running to challenge Jones, including Moore and current Alabama Secretary of State John Merrill. Experts say Jones's strongest chance of holding his seat will come if Moore ends up as the Republican nominee.

  • Georgia. Though she narrowly lost the state's voter-suppression-plagued 2018 gubernatorial election to Republican Brian Kemp, Democrat and longtime voting rights advocate Stacey Abrams showed the power of organizing and inspiring a new electorate and fighting to ensure that every vote is counted. Mentioned as a possible 2020 Senate candidate, Abrams has chosen not to run, but the momentum built up around her campaign gives Democrats hope. While Georgia has only one regularly scheduled Senate election in 2020, for the seat that will be defended by first-term Republican David Perdue, his fellow Georgia Republican U.S. Sen. Johnny Isakson recently announced that he would step down at the end of this year due to health issues. His term was scheduled to end in 2022. Kemp will appoint someone to the seat until a special election is held next year to fill it until the regular election. When a state elects two U.S. senators in the same year, history shows that one party typically takes both seats. So far, seven Democrats are running to go up against Perdue; they include Jon Ossoff, who drew national attention when he ran for Georgia's 6th Congressional District in 2017 in a special election that he lost to Republican Karen Handel. While U.S. Rep. Doug Collins, a leading Trump defender in Congress, has expressed interest in being appointed to the Isakson seat, no Democrat has announced plans yet to run next year.

  • Kentucky. Despite the fact that McConnell has a low approval rating in his state and faces a primary challenge from former state Rep. Wesley Morgan, his Senate seat is considered "safe" — but Democrats are not giving up. Among the five Democrats running to go up against McConnell is Amy McGrath, the retired Marine fighter pilot who narrowly lost her bid for Kentucky's 6th Congressional District in 2018. On the first day of her Senate campaign, she raised over $2.5 million, showcasing just how much enthusiasm there is for unseating McConnell. Among the efforts targeted at unseating McConnell is "Get Mitch or Die Trying," a campaign launched by Crooked Media, a firm founded by former Obama staffers that's also backing Democrats in other key states including Georgia, North Carolina, and Texas.

  • North Carolina. For over 130 consecutive weeks, social justice activists in Republican U.S. Sen. Thom Tillis's home state of North Carolina have rallied against his policy positions at weekly "Tuesdays with Tillis" protests. The former state House leader has also seen his ratings slip among Republicans. These are among the reasons why Tillis is considered among the most vulnerable candidates nationwide facing re-election in 2020. Tillis faces two Republican challengers in the primary: Sandy Smith, a farm owner, and Garland Tucker, a businessman and former senior fellow of the John Locke Foundation and board member of the Civitas Institute, conservative advocacy groups founded and funded by leading GOP donor Art Pope. Tucker has gained support from some of Tillis's past donors. According to a poll released last month by Public Policy Polling that did not include Smith, 38% of those surveyed said they would vote for Tillis, 31% said they would vote for Tucker, and the rest were undecided. On the Democratic side, four people are running to challenge Tillis, including former state Sen. Cal Cunningham and current state Sen. Erica D. Smith.

  • Texas. Like Georgia, Texas also experienced a high-profile election in 2018 that showcased the power of organizing. Though Democratic former U.S. Rep. Beto O'Rourke lost his challenge to incumbent Republican Sen. Ted Cruz, the election showed that no state is inevitably red and that change can happen through engaging new voters. In 2020, it will be Republican U.S. Sen. John Cornyn who will face Texas voters. In the Republican primary, Cornyn has two challengers, while eight Democrats are running to unseat him. This week, the Democratic Party of Texas announced an extensive campaign to oust Cornyn. It aims to register 2.6 million new Texas voters, place 1,000 field organizers on the ground to mobilize voters, and boost turnout in suburban areas, where support for the Trump agenda is weak.

Flipping any of these seats will not come easy for Democrats. All of these Southern states except Kentucky have some form of voter ID law, which research has shown discriminates against racial minorities. At the same time, the South has suffered from a historic lack of investment by the Democratic Party, despite the fact that the region has been the site of some critical electoral victories and near-victories in recent years. Though Democratic Senate candidates are out-fundraising Republicans nationwide, for instance, the only Southern state where that's happening currently is Alabama. If Democrats are serious about taking the Senate, they'll have to organize, mobilize, and invest in every region.