PHOENIX — The Southwest has emerged as a key battleground for control of the United States Senate in the final days of the turbulent midterm election, with off-the-charts early voting numbers giving Democrats hope that they can win tight races in Arizona, Nevada and maybe, just maybe, the bank shot of them all, Texas.
Republicans have been dominant for years in Arizona, which has not elected a Democratic senator since 1988, and senators have a good track record at winning re-election in Nevada, where Dean Heller, a Republican, is now seeking a second term. But Nevada polls recently have shown Mr. Heller and the Democratic nominee, Representative Jacky Rosen, trading off in the lead; Arizona polls have shown a neck-and-neck race between the Democrat, Representative Kyrsten Sinema, and the Republican, Representative Martha McSally.
The mere fact that the Republican Senate candidates haven’t put away the race in those two states has been enough to lead some Democrats to think that Tuesday’s election could lead to a very late night, with the East Coast waiting for the results out West to see who controls the Senate. Republicans now have a single-seat majority.
As for Texas, there are signs that Beto O’Rourke, the Democratic nominee, has tightened his race against Senator Ted Cruz, the incumbent Republican. Still, Mr. Cruz holds a lead in recent polls, and no Democrat has won statewide in Texas since 1994.
Republican officials remain optimistic about keeping control of the Senate, while Democratic officials acknowledge that their best-case scenario for winning the chamber relies on Arizona and Nevada, which would involve a major shift in the political tectonic plates of the West.
Democratic victory in those states plus Texas would be enough to win the party control of the Senate, assuming it keeps its losses to North Dakota, a state most strategists believe is likely to fall into Republican hands. (Democrats are in tough races in Indiana and Missouri as well.) A trifecta of wins, or even victories in Nevada and Arizona with a narrow loss in Texas, would have a significant impact on the party’s strategy and message in 2020 and beyond, shifting emphasis away from white Midwestern voters and toward the fast-growing, more diverse West.
“If you look at the electoral map of 2020, you can’t rely on the blue firewall anymore,” said Ruben Gallego, a Democratic congressman who represents a Phoenix-area district. “The way you put more states on the map is essentially by adding places like Arizona.”
Strong early voting numbers for Democrats, which look more like the party’s higher turnout in presidential cycles, have buoyed party strategists, given that early voters make up a majority of the total electorate in both states. In Nevada, Democrats have a three-point edge over Republicans; in Arizona, Democrats have all but closed the traditional lead Republicans have before Election Day.
“Democrats really just two weeks ago activated some sort of turnout machine,” said Garrett Archer, a senior analyst at the Arizona secretary of state’s office.
But what remains to be seen is how Election Day turnout will shake out. Jon Ralston, a prominent political analyst in Nevada, said the Democrats had a roughly 50,000-vote firewall thanks to early voting.
“Is that predictive? Sometimes. Most of the time,” Mr. Ralston said. “But it’s not so large as it is during a presidential year that you can say for sure what’s going to happen.”
Like Democrats across the country, both Ms. Sinema and Ms. Rosen have relentlessly hammered their Republican opponents for their support of last year’s health care bills in Congress, which would have weakened protections for Americans with pre-existing conditions. They’ve also aggressively pursued Latino voters, who often cite health care as their biggest concern.
Mr. Heller and Ms. McSally, in turn, have run on the strong economy and an embrace of President Trump, who has held rallies in both states.
Across the West, the president’s divisive rhetoric has also complicated the traditional path to victory for Republican candidates in the historically libertarian region. In Arizona, strategists in both parties say the visit from Mr. Trump two weeks ago hurt Ms. McSally’s campaign, lowering her support among independents and moderate Republicans. (He has not returned since then, whereas he has made multiple visits to other battleground states.)
But the two female Democratic candidates are also running campaigns tailor-made to their states.
Ms. Rosen, who was recruited by the former Nevada Senator Harry Reid to run for the seat, has thoroughly aligned herself with the state’s robust party machine and labor groups like the Culinary Workers Union that are helping fuel turnout. Former President Barack Obama campaigned on her behalf in Las Vegas several weeks ago and Senator Kamala Harris of California and the comedian Jimmy Kimmel were with her over the weekend.
Ms. Sinema, an antiwar Green Party activist who’s become a moderate House Democrat who votes with Mr. Trump nearly half the time, has presented herself as an unbeholden voice for a state where independents make up the largest voting block. She has said she won’t support the Senate minority leader, Chuck Schumer, if she wins.
“Arizonans are very independent people. They care less about party titles and labels and ideology and more about who’s going to stand up for them,” Ms. Sinema said in an interview. “The two political parties have not served their constituents very well.”
Democratic organizers in Nevada are hoping that expansive canvassing efforts, bread-and-butter messaging and antipathy toward Mr. Trump will appeal to their coalition after disastrous turnout in the 2014 midterms. On television in the Las Vegas area, Spanish-language ads run constantly accusing Mr. Heller of turning his back on his constituents.
The efforts in Nevada have been bolstered by the state’s labor unions, which have thrown their considerable weight and grass-roots organizing power behind Democrats down the ticket. The Culinary Workers Union, which has 57,000 members, estimates it will have knocked on nearly 400,000 doors by the time polls close Tuesday, according to a spokeswoman. The union, which is majority minority and 54 percent Latino, has registered more than 10,000 people to vote since the 2016 election.
“I think they’re in triage mode. We’re in don’t-step-off-the-gas mode,” said Megan Jones, a Democratic political strategist in the state.
Mr. Heller’s campaign declined repeatedly to provide The New York Times with a public campaign schedule, and one did not appear to be available online.
“We are confident going into Election Day,” a spokesman, Keith Schipper, said in an emailed statement.
Nevada Republicans see turnout in rural areas and absentee ballots as important counterbalances to any “Democratic firewall.”
Andy Abboud, a spokesman for the billionaire casino magnate and major Republican donor Sheldon Adelson, said he believed that Republicans had more highly reliable voters who will show up on Election Day.
“But it’s going to be a very long night,” he added.
In Arizona, Ms. McSally has sought to unify her party around her candidacy after a divisive primary. She has hit every corner of the Republican base, bringing in a swath of surrogates including Mitt Romney, the former presidential nominee turned Utah Senate candidate, and Donald Trump Jr., the president’s son.
Standing on a small podium set up in a shopping mall food court in the Republican stronghold of Prescott on Sunday morning, Ms. McSally cast Ms. Sinema as “out of the mainstream and out of touch with Arizona and American values.”
Ms. Sinema spent part of Sunday hugging her way through the crowd at the St. Mary’s Basilica Dia de los Muertos festival. “You know what we’re going to do on Tuesday? Win,” she whispered to a young boy, before turning her attention to a family done up in matching skeleton face paint.
A day earlier, in Scottsdale, Ariz., a steady stream of voters came to cast ballots at a civic center, walking past shirtless volleyball players and picnicking families enjoying the sunny afternoon.
Sandy Kenger, 59, who was dropping off an absentee ballot for her 91-year-old mother, said she’d recently switched her voter registration from independent to Democrat. Both Ms. Kenger and her mother voted for Ms. Sinema.
“I’ve lived here since I was 8 and it feels like there’s a political shift,” Ms. Kenger said. “For me, I was like, ‘I don’t want to align with any party’ but then it was like, I will never align with the Republican Party the way it is right now.”