WASHINGTON — Pinned six years in the minority, Democrats have an uphill but real shot at wresting Senate control in January, with more opportunities in 2022. Yet as states increasingly sort themselves along hardening partisan lines, it’s complicating Democrats’ drive to win the majority and keep it.

Thanks to this month’s elections, Democrats will own all four Senate seats from purple Arizona and increasingly blue Colorado next year. If they can win January runoffs for both seats from Georgia, which has recently teetered toward them, they’ll command the Senate thanks to Vice President-elect Kamala Harris’ tie-breaking vote in what would be a 50-50 chamber.

Yet even as Democrats have made those gains and others since surrendering control in the 2014 elections, they’ve lost foundations of their old majority that will be hard to recapture. Gone are seats from Alabama, Alaska, Arkansas, both Dakotas, Florida, Indiana, Iowa, Louisiana, Missouri, North Carolina and West Virginia, all of which tilt Republican in presidential elections.

In addition, three current Senate Democrats are from states that President Donald Trump carried easily this month despite losing to Democrat Joe Biden. Sens. Joe Manchin, 73, of West Virginia, Jon Tester, 64, of Montana and Sherrod Brown, 68, of Ohio are all proven brand names in states that would be hard for Democrats to hold without them.

All this means a constricted playing field for Democrats to add seats in coming election cycles. The same is true for Republicans, but it’s on Democrats to gain ground and keep it if they want to control a Senate that, with the Democratic-led House, could make Biden’s legislative agenda more ambitious.

“The Democratic caucus for a long time was built on winning races in red states” where they’ve since lost, said Matt House, a former Senate Democratic leadership aide. “The problem is a Democratic Senate majority runs through red states, and that is an inherent structural difficulty.”

Nothing is set in stone in politics, where momentum and issues can shift abruptly. Besides Georgia, Democrats hope to grab Senate seats soon in purple North Carolina, Pennsylvania and Wisconsin, plus Texas as that state’s Hispanic population grows. GOP Sens. Richard Burr of North Carolina and Pat Toomey of Pennsylvania have said they won’t seek reelection in 2022.

“A few years ago, people would have laughed at the idea of two Democratic senators from Arizona,” said Sen. Chris Van Hollen, D-Md., former head of the Democratic Senatorial Campaign Committee, the party’s Senate campaign arm. He said Biden’s message of bringing people together will be a “potential strength in some of these states” for Democratic candidates.

That Pattern of Partisan Allegiance Largely Persists Into the Future

Even so, recent elections underscore how states have formed into solid partisan columns as voters’ ticket-splitting becomes increasingly rare.

Setting Georgia’s January elections aside, in every state but six, both senators will be from the same party next year. A seventh, Vermont, is represented by a Democrat and independent Bernie Sanders, who is aligned with them.

In addition, every state represented by two senators from the same party was carried by that party’s presidential candidate this year, Biden or Trump.

That pattern of partisan allegiance largely persists into the future. In the 2022 elections, Democrats will defend Senate 13 seats — all from states Biden won. Trump won 18 of the 21 seats Republicans will protect. Biden won two others, and The Associated Press hasn’t yet called the presidential winner in Georgia.

“We can win all across the country” in 2022, said Sen. Rick Scott, R-Fla., who will lead the National Republican Senatorial Committee beginning next year. He added that without Trump on the ballot to draw GOP voters, “We’ll have to really hustle.”

Part of Democrats’ challenge in Senate elections is their voters tend to concentrate in coastal states. Republicans often rely on that as a campaign theme, accusing Democrats of catering to liberal coastal elites.

“Democrats are focused on the coasts and spend a lot of time flying over the bulk of the country and talking right past it,” said Scott Reed, a Republican strategist.

In one manifestation of that imbalance, more people voted for Democratic than GOP Senate candidates in 2018 by 51 million to 33 million, according to data compiled by The Associated Press. Republicans still gained two seats.

With votes still being counted in this year’s elections, Republican Senate contenders have so far garnered more votes than Democrats, 42 million to 37 million, according to an AP count.

Yet as Biden Prepares to Take Office, There’s a Warning Sign for Democrats

The difference between the two elections: seats from huge Democratic states like California and New York were on ballots in 2018 but not this year. Around one-third of the Senate’s seats are at stake each election.

Whatever happens in Georgia in January, Democrats have high hopes for 2022. Besides defending fewer seats than Republicans, only three Democratic seats seem potentially competitive. The 21 seats Republicans must protect include Burr’s in North Carolina and Toomey’s in Pennsylvania, plus a competitive race in purple Wisconsin.

Yet as Biden prepares to take office, there’s a warning sign for Democrats. Since Ronald Reagan’s presidency, the party in control of the White House has gained Senate seats in only three midterm elections: 2018, 2002 and 1982.

Things brighten for Republicans in 2024, when they’ll defend just 10 seats compared to Democrats’ 23. Democratic seats in play that year include those of Manchin in West Virginia, Montana’s Tester and Brown’s in Ohio.

“If Democrats are going to compete in red states, they’re going to have to find candidates as uncomfortable with the Democratic brand as voters in those states are,” said Brad Todd, a GOP consultant.

Democrats’ prospects could be further complicated after two years in which the moderate Biden pushes an agenda as progressives press him on issues like climate change and health care, dividing the party.

“You can still win red states if you recruit good candidates who are unique fits for their state,” said Brian Fallon, a former Senate Democratic aide who runs Demand Justice, a progressive group. He said that requires mobilizing the party’s multi-ethnic liberal voters while curbing the loss of less-educated white voters, which he called “a unique challenge.”