While all eyes are on next year's presidential race, both parties will be doing all they can to take the Senate.

Republicans currently control the Senate by a six seat margin, so Democrats would need to pick up three seats if President Trump loses reelection and four seats if he wins in order to flip control. There are 34 seats up for reelection in 2020, and 22 of them are held by Republicans.

Defending 22 seats sounds like a daunting task, but only two of those seats are in states Hillary Clinton carried in 2016. And there are even potential pickups for Republicans in states with Democratic incumbents. Of the 12 seats Democrats must defend, one is in Alabama, which Mr. Trump won handily in 2016, and two are in swing states: Michigan and New Hampshire.

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Below are some of the races that could decide who wins the majority in 2020.

Colorado

Incumbent: Cory Gardner (Republican)

Sen. Cory Gardner is facing an onslaught of challengers in Colorado where 11 Democrats have already filed to run against him. Gardner's challenge will be navigating how close he should hew to the president. Hillary Clinton won the state by five percentage points in 2016, and Mr. Trump's current approval rating in Colorado is 42 percent, according to Morning Consult, down 14 points since he took office.

In 2018, the state trended blue. Democratic candidate Jason Crow defeated incumbent Republican Mike Coffman for a seat in the U.S. House of Representatives, and Jared Polis won the governor's race by over 10 percentage points to become the first openly gay governor in the country.

Gardner ran in 2014 as a moderate but has since become an ally of Mr. Trump. The Denver Post's editorial board withdrew its endorsement of Gardner when he voted for the president's national emergency resolution in March. The board criticized him for valuing professional security over the interests of the state. But in recent months, Gardner has also proposed several bipartisan bills he can run on in 2020 including one to expand public lands access and another to expand broadband coverage to the rural parts of Colorado.

The top tier of candidates running against Gardner, according to strategists, consists of former state Sen. Michael Johnston, former state Rep. Andrew Romanoff, and Dan Baer, former ambassador to the Organization for Security and Cooperation in Europe under President Obama. And there are still more Democrats who could get in ahead of the March 2020 filing deadline. Some contenders Democrats have mentioned are Jena Griswold, the Colorado secretary of state; Jason Crow, the new congressman; and John Hickenlooper, the former governor who is currently running for president.

Arizona

Incumbent: Martha McSally (Republican)

In Arizona, Sen. Martha McSally will have to win the seat Republican Gov. Doug Ducey appointed her to in 2018 after she lost to Kyrsten Sinema by less than three percentage points. Ducey appointed McSally in December after John McCain's chosen successor Jon Kyl announced he would step down.

To win in 2020, McSally's team will have to figure out why Arizonans voted for Ducey but not for McSally last November, and why she lost the suburban vote in Maricopa County, which contains at least 58 percent of the state's registered voters. Phoenix, in the middle of the county, is the fastest growing U.S. city, according to the U.S. Census Bureau, and is projected to have more than 140,000 more Hispanic voters in 2020 than in 2018, according to the Arizona Office of Economic Opportunity.

McSally, however, is likely to have more time this cycle than she did in 2018 to build her general election support. In 2018, she faced a contentious primary fight with conservative challengers Joe Arpaio and Kelli Ward in August and then had to turn around and face Sinema in November. And she has already been active this year on legislation she can campaign on in 2020.

In early May, McSally introduced a bill that would reimburse state and local governments for immigration costs. The bill has received support from Arizona's law enforcement, a key constituency in the state. She has also been advocating for other bills like one to address military sexual assault and the Colorado River Drought Contingency Plan Authorization Act, which would ensure there is a plan in case of a drought in Arizona.

Democrats will likely hammer McSally for her closeness with Mr. Trump, particularly when it comes to health care. In the Senate, she has voted with Mr. Trump 95.4 percent of the time, according to FiveThirtyEight.

Mr. Trump defeated Clinton in 2016 by just over three percentage points in Arizona, and since his inauguration his net approval rating has decreased by 28 percentage points, making the state a legitimate target for Democrats in 2020.

Former astronaut Mark Kelly, who is also the husband of former Rep. Gabby Giffords, announced his candidacy in February and outraised McSally in the first quarter, according to the Federal Election Commission. Kelly has raised over $4 million and McSally raised $2 million.

Maine

Incumbent: Susan Collins (Republican)

Sen. Susan Collins is receiving national attention in her bid for reelection in Maine because of her game-changing vote to confirm Brett Kavanaugh to the Supreme Court. In response to the Kavanaugh vote, pro-abortion rights activists have crowdfunded almost $4 million for her eventual Democratic opponent.

However, no one has stepped up to the plate yet. Collins hasn't faced a competitive race since she was elected in 1996. She carried every county in Maine during her runs for reelection in 2002, 2008, and 2014. Democratic strategists expect Sarah Gideon, the speaker of the Maine State Legislature, to file her to run shortly after the state's legislative session ends on June 19th. The other name floating around is Hannah Pingree, the daughter of Maine Rep. Chellie Pingree. Former National Security Advisor Susan Rice threatened to run after Collins' vote on Kavanaugh but has since ruled it out.

The success Democrats had in 2018 makes the state a target for the Democratic Senatorial Campaign Committee. During the midterms, Democrat Jared Golden flipped the second congressional district, and Democrats won the State Legislature for the first time since 2009. This year, the legislature has passed progressive policies such as a paid leave program and two bills expanding access to abortions.

Collins is working on legislation to appeal to Maine's older population. Maine has the second largest population of citizens ages 65 and older, right behind Florida, according to the U.S. Census Bureau. She authored an amendment that passed in the Senate that would create a panel on seniors and disasters to prepare older Americans for a disaster and introduced a bill to support grandparents who have become primary caregivers for children exposed to substance misuse or other trauma. Her reelection campaign has received over $4 million this year so far, according to FEC reports.

North Carolina

Incumbent: Thom Tillis (Republican)

Sen. Thom Tillis is facing a reelection battle in a state Mr. Trump won by three percentage points over Clinton. Tillis' path to reelection is more complicated because he already has a Republican primary challenge from North Carolina businessman Garland Tucker, and the conservative Club for Growth has floated Rep. Mark Walker's name as another potential challenger. If Tillis doesn't attain at least 40 percent of the vote in the primary, he'll have to spend a chunk of cash to compete in runoff race before advancing to the general election.

Tillis has attracted challenges in part because of his flip-flop on Mr. Trump's national emergency bill. He wrote an op-ed in the Washington Post criticizing the declaration and then ending up voting for it. The move angered Trump supporters as well as moderates in the state. Democrats are still recruiting top tier candidates who can fundraise enough money to compete with Tillis who will likely be receiving support from the National Republican Senatorial Committee (NRSC) and the Trump campaign. Two of the potential Democratic contenders are former state treasurer Janet Cowell and State Senator Jeff Jackson.

Tillis may benefit from the attention Republicans will give the state in 2020 because of the presidential election, the Senate race, the governor's race, and the Republican National Convention. And in 2019, the state will have a redo for North Carolina's ninth congressional district which was recalled after allegations of ballot tampering in favor of the GOP. The race will provide Republicans with a test case for how to approach all of the 2020 races.

Alabama

Incumbent: Doug Jones (Democrat)

Sen. Doug Jones is facing a reelection battle in ruby red Alabama. Mr. Trump won the state in 2016 by almost 30 percentage points, and Jones only beat Roy Moore in 2017 by 1.7 percentage points despite accusations against Moore of inappropriate conduct with women. Mr. Trump's approval rating in the state according to Morning Consult is at 61 percent, and Republicans won all but one statewide race in 2018.

Jones is working in the Senate on appealing to moderates in Alabama. He has voted against the Democratic party over 22 percent of the time, the third highest percentage in the Senate, according to ProPublica.

Some of these votes have been for judges the Republican majority is nominating to the courts. Just in May, he voted to confirm four judges that the majority of Democrats in the Senate opposed.

The 2017 special election that Jones won was to fill Jeff Sessions' seat when he left Congress to become Mr. Trump's first attorney general. There are several Republicans who have already filed to challenge Jones, including Rep. Bradley Byrne and former Auburn football coach Tommy Tuberville.

To national Republicans' dismay, Moore has indicated he's interested in running again for the Senate seat. Both Mr. Trump and his oldest son, Donald Trump Jr., have tweeted that Moore should stay out of the race.

Michigan

Incumbent: Gary Peters (Democrat)

Michigan was key to Mr. Trump's victory in 2016, but so far, no Republican candidates have filed to challenge Democratic incumbent Sen. Gary Peters.

In 2016, the state voted for a Republican presidential candidate for the first time since 1988. Mr. Trump beat Clinton by 0.3 percentage points. But since his inauguration, Mr. Trump's approval rating has decreased by 18 percentage points in the state, according to Morning Consult. And in 2018, Democrats picked up the governor's seat and two House districts.

The Republican National Committee is focusing resources in an effort to keep it in Mr. Trump's corner and has recently hired a state director for Michigan. And Peters remains relatively obscure to voters, with recent poll from Morning Consult showing that Peters is the most unknown Senator facing reelection in 2020.

But Peters has an edge when it comes to fundraising. In the first quarter of 2019, Peters raised over $4 million, according to reports submitted to the FEC.

On the Republican side, Detroit businessman and combat veteran John James might run as a Republican for Senate again. In 2018, he lost to incumbent Democratic Senator Debbie Stabenow by just over six percentage points. Both the GOP's Senate and House campaign committees are encouraging James to run in 2020 against either Peters or Rep. Haley Stevens, who flipped a House seat away from Republicans in 2018. Both Peters and Stevens' campaigns have sent fundraising emails in recent weeks about a potential threat from James.

New Hampshire

Incumbent: Jeanne Shaheen (Democrat)

Clinton won New Hampshire by less than one percent in 2016, so Republicans are focusing on the state as a potential pickup opportunity for Mr. Trump in 2020 as well as a Senate battleground.

Democratic incumbent Jeanne Shaheen only narrowly won her race over Scott Brown in 2014. Despite the opportunity, no Republicans have filed to challenge her yet, but potential recruitments include former New Hampshire House Speaker Bill O'Brien and retired Brigadier Gen. Don Bolduc, who was the commander for Special Operations Command Africa from 2015 to 2017. Republican Gov. Chris Sununu announced in May that he would forgo a Senate bid and instead run for reelection.

Republicans would have to recruit a strong candidate because despite scraping away with a victory in 2014, Shaheen has the second highest approval rating of Senators up for re-election, according to Morning Consult, and FEC reports show she raised over $3 million in the first quarter of this year. In an attempt to chip away at her, the GOP released ads in May attacking her for her support of Sen. Bernie Sanders' Medicare-for-All proposal.

Shaheen will likely run on the legislation she has supported. For instance, she has co-sponsored a bill to increase higher education institutions' accountability for ensuring students can repay their student loans. The Institute for College and Success reported that New Hampshire has the fourth highest average student loan debt in the country with $34,000.

Shaheen also introduced provisions in the bipartisan bill to address the opioid epidemic -- a major issue for New Hampshire -- that passed and was signed into law by Mr. Trump in October of 2018. Her provisions included prioritizing funding for states that are hardest hit and introducing peer recovery networks.