A generous Senate map in last week's midterm elections helped Republicans keep a majority in the chamber even as they lost control of the House.
The GOP will not have such an easy path in 2020.
So far, Republicans have unseated three Democratic senators in this year's midterms, while Democrats have won two GOP-held Senate seats, according to projections. Depending on how a recount in Florida and runoff in Mississippi play out, Republicans could hold their current 51 seats or expand their majority to 53 seats in January. The election could have gone worse for Democrats: 26 Democratic senators and independents who caucus with them faced re-election this year, while only nine Republican-held seats were up for grabs.
The parties will battle on a more balanced map two years from now. The GOP is expected to defend 22 seats, including a special election for the late Sen. John McCain's office, while Democrats are set to face re-election in 12 states.
As the GOP will likely expand its majority when all the 2018 ballots are counted and most of the senators running in 2020 appear safe, Democrats still have a difficult path to a majority. Both parties have a few good opportunities to unseat incumbents in the next congressional elections.
Here are the seven Senate seats most likely to flip in 2020, although the dynamics could change depending on whether all incumbents seek re-election. The graphics include margins of victory for recent presidential and Senate races in the states.
Jones won a special election last year to replace Republican Jeff Sessions, who served as President Donald Trump's attorney general until last week. The Democrat will likely have a tough time winning again in the regular election for Sessions' seat.
Jones defeated Republican Roy Moore, the pro-Trump former chief justice of the Alabama Supreme Court. The GOP candidate faced accusations of sexually abusing teenagers decades ago, which led to fewer voters turning out for him than they would for a typical Republican.
Trump won the state by about 28 percentage points in 2016. Sessions ran uncontested in 2014.
Gardner faces a fight for his political life in a state that appears increasingly blue. Since 2008, Democrats have won every Senate, gubernatorial or presidential race in Colorado — except for Gardner's victory over Democratic Sen. Mark Udall in 2014.
Hillary Clinton won the state in 2016 by about 5 percentage points as Democratic Sen. Michael Bennet comfortably won re-election. Last week, Democrat Jared Polis won the Colorado gubernatorial race by about 8 percentage points.
Gardner, a first-term senator, rarely broke with his party on its top priorities during the Trump administration, despite his state's blue lean. He voted with Republicans on a range of key measures, including repeal of the Affordable Care Act, the overhaul of the U.S. tax code and the confirmation of Justices Neil Gorsuch and Brett Kavanaugh to the Supreme Court.
However, he did buck the GOP on major issues such as marijuana and immigration. He held up Justice Department nominations in order to push the Trump administration to protect states that legalized marijuana, and worked with Democratic Sen. Elizabeth Warren on a marijuana reform bill, among other efforts.
Republicans just lost a Senate election in Arizona for the first time in decades, with Democrat Kyrsten Sinema's apparent victory over Republican Martha McSally. The GOP could soon lose another.
After Sen. John McCain's death, Republican Gov. Doug Ducey appointed former GOP Sen. Jon Kyl to replace him in September. However, Kyl has indicated he may not serve past early next year, which would require another Republican to take his seat and potentially run at the end of McCain's term in 2020.
Whoever it is — McSally has been mentioned as a candidate — will have to win in a state where Democrats have enjoyed increasing success. Trump won the state in 2016 by only about 3 percentage points, while Sinema now leads by 1.7 percentage points with some votes outstanding.
In Maine, Collins will likely face resistance on two fronts if she decides to run for re-election in 2020. Republicans have criticized the fourth-term senator for her vote against Obamacare repeal. Democrats have slammed her for her support of Kavanaugh.
Democrats won the 2016, 2012 and 2008 presidential elections in Maine. Sen. Angus King, an independent who caucuses with Democrats, easily won re-election there last week. And Democrats have already crowdfunded nearly $3 million to support Collins' eventual Democratic challenger.
But do not count Collins out. She has cruised to victory in three of her four Senate elections, winning by more than 30 percentage points in 2014.
North Carolina has not voted for a Democratic presidential or Senate candidate since President Barack Obama and former Sen. Kay Hagan in 2008. Still, first-term Republican Tillis could face a tough challenge in 2020.
Democratic Gov. Roy Cooper narrowly won the state in 2016, which gives Democrats hopes about prevailing statewide again. His triumph came even as Trump beat Clinton by about 3 percentage points and GOP Sen. Richard Burr won re-election by about 6 percentage points.
Tillis, like Gardner, has not broken from his party on its major policy priorities despite North Carolina's swing state status.
New Hampshire's two-term Democratic senior Sen. Jeanne Shaheen was first elected in 2008 in the once reliably Republican state to become the first Democrat elected to the office form the Granite State since 1980. In 2014, she became only the second Democrat from New Hampshire to be re-elected to the Senate, and the first since 1972.
She is also the first woman to represent New Hampshire in the Senate, having served earlier as the first female governor of New Hampshire.
Despite her solid record of popularity with New Hampshire voters, the state remains closely divided between upstate Republicans and more Democratic voters in Boston's outer suburbs. Clinton won the 2016 presidential vote by a razor-thin 0.4 percent margin, the second closest percentage after Michigan.
Ernst, Iowa's Republican junior senator, is a former state senator and a veteran of the Iowa Army National Guard, retiring as a lieutenant colonel. She's the first woman to represent Iowa in Congress and the first female combat veteran elected to the Senate from any state.
Ernst was first elected in 2014 to the seat held by retiring Democratic Sen. Tom Harkin, who easily won his last re-election in 2008. While Iowa voted to elect Obama president twice, the state went for Trump in 2016 by more than 9 points. That year, seven–term incumbent Republican Sen. Chuck Grassley easily won re-election.
But Democrats in Iowa posted solid gains in this year's midterm elections, flipping two of the three GOP-held House seats from red to blue. Iowa's fourth congressman, seven-term Democratic Rep. Dave Loebsack, easily won re-election.