President Donald Trump's fight to hold the White House will captivate America next year. But don't forget about the Senate Republicans the president relies on to push his nominees and policy goals through Congress. The GOP holds a 53-47 majority in the chamber. The party's Senate control is critical for Trump, as it cushions him against the movements of the Democratic-held House. The landscape may shift in a hurry during the November 2020 elections. Not only could Trump's bid for a second term fail, but also both chambers of Congress may change hands. Republicans will have to scrap to keep their Senate majority: the party has to defend 22 seats next year, while only 12 Democratic-held seats are up for grabs. To take control of the Senate, Democrats would need to defeat GOP incumbents in some combination of states such as Colorado and Arizona — which have trended more blue in recent elections — and more reliably red states including Georgia and Texas. Democrats — including perhaps the most vulnerable senator up for reelection in 2020, Doug Jones of Alabama — will also have to defend their own seats. Here are the most important Senate races of 2020 and where they stand now in their early stages:

Alabama, Democratic Sen. Doug Jones:

Democratic Alabama U.S. Senate candidate Doug Jones and wife Louise acknowledge supporters at the election night party in Birmingham, Alabama, December 12, 2017. Marvin Gentry | Reuters

In squeaking out a 2017 special election win in deep red Alabama, Democratic Sen. Doug Jones of Alabama had the benefit of facing a flawed Republican candidate who lacked party support. Jones' bid to win a full Senate term could depend in no small part on whether he again faces former Alabama Supreme Court Chief Justice Roy Moore. Jones beat Moore — who faced accusations of sexual misconduct with teenagers decades ago — by about 1.5 percentage points in December 2017. But defending his seat in a higher turnout presidential election year in a state Trump carried by nearly 30 percentage points will prove a tougher task for the senator. His political survival may come down to who he faces next year. GOP U.S. Rep. Bradley Byrne — a mainstream Trump-era Republican who supports the president's goals on immigration and other key issues — has already entered the Senate race. Byrne would likely pose a bigger challenge for Jones than Moore, who has also suggested he could enter the race. The former judge has a chance of winning the Republican Senate nomination again: he leads the GOP primary field with 27% of support, according to a Mason-Dixon Polling & Strategy survey released Tuesday. Jones has toed the line between backing Democratic priorities and trying not to alienate voters in a state where Trump has an approval rating above 60%. He has bucked his party on recent votes to confirm Trump's nominees and on abortion-related legislation. On the other side, Jones voted to overturn Trump's national emergency declaration at the U.S.-Mexico border and end U.S. military support for a Saudi-led military coalition in Yemen. Asked where they stand on the 2020 Senate election, 40% of Alabama voters said they would vote to re-elect Jones, versus 50% who responded that they would replace him with a generic Republican, according to the Mason-Dixon poll. Jones and Byrne ended the first quarter with a similar standing in the money race. The Democratic senator raised $1.6 million during the period and had $3.1 million on hand as of March 31, according to Federal Election Commission records. The Republican representative reported $2.1 million in receipts and had $2 million in the bank at the end of the quarter.

Arizona, Republican Sen. Martha McSally:

Martha McSally, Republican U.S. Senate candidate from Arizona, speaks during an election night rally in Tempe, Arizona, on Tuesday, Aug. 27, 2018. Caitlin O'Hara | Bloomberg | Getty Images

McSally, a former U.S. representative, was the second senator appointed to Republican John McCain's seat following his death last year. She will run next year to fill the remainder of McCain's term through 2022. McSally comes off a roughly 2.5-percentage point loss to Democratic Sen. Kyrsten Sinema in Arizona's 2018 Senate race. She hopes to do better this time in the state, which Trump won in 2016 but has become increasingly competitive. Retired astronaut Mark Kelly, the husband of former Rep. Gabby Giffords, is running for Senate as a Democrat. When Democratic Rep. Ruben Gallego declined to run, it effectively set next year's matchup as McSally vs. Kelly. McSally has largely aligned with Trump during her three months in the Senate. While she supported the president's national emergency declaration to secure funds for his proposed border wall, she opposed his recent threats to shut down the border with Mexico. The move would damage businesses in border states such as Arizona and Texas, and strategists say it could hurt incumbents running in those states next year. In launching his campaign, Kelly said he would focus on issues such as health care and climate change. He has also pushed for gun safety rules after a shooting at a public event left Giffords with a severe brain injury. Kelly enjoyed strong fundraising in the first quarter. His campaign reported $4.1 million in total receipts, and ended March with $3.2 million on hand, according to the FEC. McSally's campaign raised $2.1 million in that span and reported having $2.1 million in the bank at the end of the first quarter.

Colorado, Republican Sen. Cory Gardner:

Sen. Cory Gardner Bill Clark | CQ Roll Call | Getty Images

Democratic Colorado Gov. Jared Polis comfortably won election in 2018. Democrat Hillary Clinton won the state by about 5 percentage points in 2016, while Democratic Sen. Michael Bennet carried it by a wider margin that year. Gardner hopes he can reverse the GOP's recent misfortune in Colorado to win re-election in 2020. The Republican senator has carved out an independent streak on issues such as immigration and marijuana legalization. Still, he opposed blocking Trump's national emergency declaration even as 12 Republicans voted to reverse Trump's action last month. The field vying to take on Gardner has already started to fill up. Former Obama administration official Dan Baer, former U.S. attorney John Walsh, former state Sen. Mike Johnston and former state House Speaker Andrew Romanoff have entered the race. Gardner has a head start on the cash front. His campaign reported $2 million in receipts from January through March, and ended the first quarter with $3.4 million on hand.

Georgia, Republican Sen. David Perdue:

Sen. David Perdue (R-GA) speaks as (L-R) Sen. Todd Young (R-IN), Sen. Dean Heller (R-NV), Sen. Shelley Moore Capito (R-WV), Sen. Michael Rounds (R-SD) and Secretary of the Treasury Steven Mnuchin listen during a news conference on tax reform November 7, 2017 on Capitol Hill in Washington, DC. Getty Images

After Democrat Stacey Abrams fell just short of winning the Georgia governor's office last year, Democrats hope they can top Perdue and Trump statewide in 2020. The party has a challenge ahead. Georgia last had a Democratic senator in 2005. A Democratic presidential nominee has not won the state since 1992. Abrams may once again enter the fray in 2020. The former Georgia House minority leader — who delivered Democrats' response to Trump's State of the Union this year — is considering challenging Perdue. No major Democrats have entered the race yet. Perdue has reliably voted with Senate Republicans during Trump's presidency. Abrams, when running for governor, made health care her signature issue. She pushed for Georgia, one of 14 states not to approve Medicaid expansion under the Affordable Care Act, to widen access to the federal and state health insurance program for low-income Americans. Perdue won his first Senate election in 2014 by about 8 percentage points. Abrams lost to Republican Georgia Gov. Brian Kemp by about 1.5 percentage points last year. Perdue's campaign took in $1.8 million in the first quarter, ending the period with $3.3 million in the bank.

Maine, Republican Sen. Susan Collins

Sen. Susan Collins (R-ME) speaks to reporters as she arrives for a nomination vote on Capitol Hill in Washington, December 13, 2017. Joshua Roberts | Getty Images

Collins found herself in the middle of two of the most pivotal moments of Trump's presidency. She irritated conservatives with one key vote, then enraged liberals with another — making her reelection bid one of the most closely monitored in the country in 2020. Election watchers have raised the prospect of not only a competitive general election, but also a potential primary challenge. Collins is among the Senate Republicans who breaks with her party most often. She cast one of the three Republican votes that shot down her party's attempts to repeal the Affordable Care Act. Since February, she voted to terminate Trump's emergency declaration, end U.S. support for the Yemen conflict and oppose Andrew Wheeler's nomination to head the Environmental Protection Agency. But Maine Democrats will likely only remember one vote. In October, Collins voted to confirm conservative Supreme Court Justice Brett Kavanaugh. The Senate narrowly put Kavanaugh on the top U.S. court, tilting its ideological balance, after he faced several sexual misconduct accusations. He vehemently denied the allegations. No specific Democrats have announced plans to challenge Collins. Still, she is one of the senators Democrats most want to unseat next year: a crowdfunding campaign for the eventual Democratic nominee set up after Collins' vote for Kavanaugh raised more than $3 million. Collins' campaign reported $1.5 million in total receipts from January through March. It had $3.8 million in cash on hand at the end of the first quarter.

Michigan, Democratic Sen. Gary Peters:

Sen. Gary Peters, D-Mich. Bill Clark | CQ-Roll Call Group | Getty Images

Peters is not considered one of the most vulnerable senators running for reelection next year. Still, he will try to hold his seat in a state Trump won in 2016, which makes the race intriguing with the president again on the ballot. Trump carried Michigan by fewer than 11,000 votes and may need to win the state again to hold the White House. Still, Peters has not voted like a lawmaker fearing for his political future — he has aligned with his Democratic colleagues on just about all major issues. Michigan was one of several states that shifted toward Democrats in last year's midterms after Trump carried them in 2016. Democratic Gov. Gretchen Whitmer won by nearly 10 percentage points, while Democratic Sen. Debbie Stabenow had a nearly 7-percentage point margin of victory. Trump has focused on winning the state: he already held a campaign rally there last month. It is unclear now who will challenge Peters on the Republican side. Peters' campaign took in $1.9 million in the first quarter and ended March with $3.1 million on hand.

New Hampshire, Democratic Sen. Jeanne Shaheen:

U.S. Senator Jeanne Shaheen (D-NH) seen speaking during a program titled 'Tracking Federal Funding to Combat the Opioid Crisis' at the Bipartisan Policy Center in Washington, DC. Michael Brochstein | SOPA Images | LightRocket | Getty Images

Shaheen's race — like Peters' — carries intrigue mostly because the presidential contest in her state could be close. Clinton won New Hampshire, a traditional swing state, only by a few thousand votes in 2016. Democratic Sen. Maggie Hassan defeated Republican incumbent Kelly Ayotte by about 1,000 votes that year. Shaheen won her last election in 2014 by about 3 percentage points over former GOP Sen. Scott Brown. During Trump's presidency, she has voted mostly along the Democratic Party line. Republican New Hampshire Gov. Chris Sununu is reportedly a potential challenger to Shaheen. The senator ended the first quarter with $1.5 million on hand.

North Carolina, Republican Sen. Thom Tillis:

Sen. Thom Tillis, R-N.C., attends a Senate Veterans' Affairs Committee hearing in Russell Building titled 'Fiscal Year 2020 Budget for Veterans' Programs and Fiscal Year 2021 Advance Appropriations Request,' on Tuesday, March 26, 2019. Tom Williams | CQ-Roll Call Group | Getty Images

Along with Collins, Tillis has annoyed both sides of the political aisle in a competitive state ahead of a tough re-election bid. The senator, a supporter of Trump's immigration agenda, first wrote that he would vote to block Trump's national emergency declaration at the border. As Republicans stewed at home, he reversed course and opposed the resolution to terminate Trump's executive action. Though no major primary challengers have entered the race, Tillis may face the prospect of a competitor running from the right. He could also have a tough path to keeping his seat in the general election. In 2016, Trump carried North Carolina by more than 3 percentage points, while Sen. Richard Burr won reelection by more than 5 percentage points. But Democratic Gov. Roy Cooper narrowly won statewide, giving Democrats bigger hopes for future success in the state. On the Democratic side, state Sen. Erica Smith and former Mecklenburg County Commissioners Chair Trevor Fuller have both launched campaigns for Senate. Tillis starts with a money advantage: he raised $1.2 million in the first quarter and ended March with $2.9 million in the bank.

Texas, Republican Sen. John Cornyn:

Sen. John Cornyn, R-Texas, arrives in the Capitol for the Senate policy luncheons on Tuesday, Feb. 26, 2019. Bill Clark | CQ-Roll Call Group | Getty Images