With the ongoing electoral drama in Florida, the Democrat Kyrsten Sinema’s come-from-behind win in the Arizona race for the U.S. Senate seat being vacated by the Republican Jeff Flake, which was confirmed on Monday night, hasn’t received the attention it deserves. Sinema’s victory was arguably the biggest win for Democrats since Donald Trump was elected. (In last December’s special election in Alabama, the Republicans ran an unelectable candidate.) It puts a different gloss on the results of the midterms. And it has important implications for 2020 and beyond.

Last Tuesday night, as the early returns came in, it seemed like the Democrats were heading for a narrow victory in the House and the Republicans were on course for a decisive win in the Senate. Donald Trump and other Republicans quickly seized on this narrative, which emerged before the number of Democratic pickups in the House became clear—the figure is now thirty-two, and may well go higher—and also before the results came in from Senate races in Nevada, where the Democratic challenger Jacky Rosen defeated the Republican incumbent Dean Heller by five percentage points, and Montana, where the embattled Democratic incumbent Jon Tester fought off a strong challenge from the Republican Matt Rosendale. Now Sinema’s triumph has been added to the mix.

Setting aside Florida, where there is a recount under way, the score in Senate-seat pickups is Republicans, 3, Democrats, 2. The G.O.P. gains all came in states where Trump scored huge victories in 2016: Indiana, Missouri, and North Dakota. In states that Trump carried by less than fifteen percentage points or that Hillary Clinton won, the pickups tally is Republicans, 0, Democrats, 2. (If the Republican Rick Scott holds on in Florida, the score will be Republicans, 1, Democrats, 2.) This is a more nuanced picture. It suggests that, though Trump can still rally his followers very effectively in deep-red (or, rather, deep-Trump) states, his polarizing style doesn’t necessarily play well in other areas that Republicans have traditionally relied on. And Arizona is one of those places.

For the past fifty years, Arizona has been a redoubt of the Sun Belt conservatism—pro-business, anti-taxes—that Barry Goldwater pioneered. Occasionally, moderate Democrats like Bruce Babbitt and Janet Napolitano have held statewide office. But Sinema is the first Democrat to be elected to the Senate from Arizona since 1988, and the first Democrat to win an open Senate seat in the state since Dennis DeConcini was elected, in 1976. The firsts don’t stop there. Sinema, a forty-two-year-old congresswoman for Arizona’s Ninth District, will also be the first female senator from Arizona, and the first openly bisexual senator from anywhere.

Her margin of victory was a narrow one—about thirty-eight thousand votes, or 1.7 percentage points—but she won fair and square. Last week, Trump cried “corruption” as Sinema caught up to and surpassed the vote tally of her G.O.P. opponent, Martha McSally, a fifty-two-year-old congresswoman, who represents Arizona’s Second District. McSally made no such claim. On the day of the election, hundreds of thousands of early votes were dropped off at polling places, and each of them had to be checked individually to make sure the signature matched the one on file. Most of these turned out to be Democratic votes. On Monday night, McSally posted a video in which she congratulated Sinema and said, “I wish her all success as she represents Arizona in the Senate.”

Like Nevada, Arizona is often cited as a state in which long-term demographic change, particularly the growing number of Latino residents, is favoring Team Blue. Right now, though, Arizona contains a lot more registered Republicans than Democrats, and Sinema’s electoral strategy reflected this fact. The demographic transition “is happening, but it’s not why Sinema won,” Andy Barr, a political consultant who has represented numerous Arizona Democrats, told me on Tuesday morning. “She won by running an extremely disciplined campaign focussing on what we call the swing demographic—college-educated women in the suburbs.”

As the Republicans sought to portray her as a tutu-wearing radical—back in 2000, she worked on Ralph Nader’s Presidential campaign—Sinema came out against two policies popular with progressives: Medicare for all and abolishing the Immigration and Customs Enforcement agency. In September, she supported a G.O.P. proposal in the House to extend the personal tax cuts that were introduced in last year’s tax-reform bill, which she opposed at the time. But she also talked a lot about traditional Democratic issues, such as health care and Social Security, and also emphasized her role in serving constituents in the Ninth District. “She portrayed herself as someone who gets things done and doesn’t get caught up in the partisan B.S.,” Barr said.

By campaigning as a moderate willing to cross party lines, Sinema attracted support from suburbanites and self-identified independents. She also exposed the fault line in the Arizona G.O.P., which is divided between the old-line Party establishment, which Flake and John McCain embodied, and a seething base of Trump supporters. Initially, McSally tried to straddle this divide, but she ended up embracing the President and his inflammatory policies. Appearing alongside him at a rally last month, she said, “America is back—and Arizona is back—thanks to the leadership of President Trump.” But McSally was defeated despite gaining Trump’s endorsement.

Looking forward to 2020, this outcome won’t be lost on strategists from both parties. In 2012, Mitt Romney carried Arizona by a healthy margin of nine percentage points. In 2016, Hillary Clinton lost to Trump by just 3.5 points, despite the fact that her campaign didn’t make Arizona a high priority until late in the campaign. “Any Democrat running for President in 2020 would be dumb not to invest early in Arizona,” Barr said.

Sinema’s triumph also sets the stage for a debate inside the Democratic Party about how to win red states in the Trump era. In Texas, Beto O’Rourke ran a barnstorming progressive campaign and came up just short. Despite alienating some progressive activists, Sinema hedged her way to the U.S. Senate. “There was some whining about that, but we were so hungry for a win that the Democratic coalition wasn’t complaining much,” Barr said. In politics, as in sports, winning covers up a lot of sins.