GLENDALE, Ariz. — The path to the White House runs straight through an elementary school playfield here, where on a recent Saturday morning a plane paid for by the Democratic National Committee drags a banner urging parents watching their kids play soccer to vote.

Those parents, and the other 4.7 million residents in Maricopa County, are at the epicenter of the battle for the White House. For the first time in decades, the largest county in this ordinarily conservative swing state has become an urgent priority for both Democrats and Republicans. No single media market has attracted more political advertising spending than have television stations in Phoenix.

Together, the voters of Maricopa County represent a microcosm of the poles dividing American politics today, from the nativist anti-immigration hardliners who have been President Trump’s most argent supporters to the growing minority communities who are finding their political power anew.

The fulcrum, the voters who will decide how this state — and the nation — tilts Tuesday, are the growing number of suburbanites in the rapidly expanding developments that have sent Maricopa County’s population skyrocketing. They come in search of inexpensive housing, good-paying jobs in the growing tech sector, and the year-round sun for golf-loving retirees.

“We don’t build up, we build out,” said Mike Noble, a Phoenix-based pollster. “Outside of the urban core of Phoenix, it’s pretty much a giant suburb.”

Four years ago, Maricopa County was the largest county in the U.S. that President Trump carried. He won here by 45,000 votes in 2016, or about 3 percentage points, in an election in which registered Republicans outnumbered registered Democrats by 150,000.

Four years later, Trump’s path to winning Arizona’s electoral votes once again is considerably more difficult. Since 2016, 200,000 new residents have moved in. The GOP’s voter registration edge has been cut by 50,000.

In the 2018 midterm elections, Sen. Kyrsten Sinema (D) won Maricopa County by 60,000 votes. This year, Democrats are contesting seven state legislative districts across the region, from the wealthy suburbs in Chandler to more working-class neighborhoods in Peoria and Glendale.

“These persuadable voters who have traditionally leaned right are now leaning left,” Noble said.

© Democrats are notching gains in Maricopa County. Image credit: Mike Noble/OH Predictive Insights

Trump has accelerated the rising blue wave here, voters and strategists say. His disruptive temperament and his constant attacks on perceived enemies both foreign and domestic have turned off many suburban voters, including some Republicans who approve of the policies he has pursued.

“I decided to give him a chance in 2016, but honestly for me I think the man has no morals, no ethics, and he wouldn’t know a lie if it slapped him in his face. I just can’t do it anymore, and I’m sad. I freakin’ hated being under President Obama’s legislations and rules, but I don’t believe our country is better now than it was before,” said Jeffrey Cox, 52, a self-described conservative Republican from Phoenix who voted for former Vice President Joe Biden, the first Democrat he has ever supported.

“I didn’t want to. I really didn’t want to,” Cox said. “I’ll take my chances with a little bit of liberalism when it comes to morality.”

David Shamblen, 63, was a registered Republican until a few years ago. This year, he voted for Biden — and for Democrats running in other races, too.

“Trump chased me away,” Shamblen said in an interview. “I went straight blue this time, just in case Trump gets in.”

No Democratic presidential candidate has won a majority in Maricopa County since Harry Truman sought reelection in 1948, when President Trump and former Vice President Joe Biden were toddlers. But polls show Biden leading here, by margins greater than his statewide advantage.

An OH Predictive Insights poll found Biden leading Trump in Maricopa by a 51 percent to 45 percent margin, twice Biden’s advantage in the statewide sample.

Maricopa represents about 62 percent of all registered voters in Arizona, and winning here is virtually essential to winning statewide. Only once in modern history has a candidate won statewide office in Arizona without carrying its largest county — and that candidate, former Superintendent of Public Instruction Diane Douglas (R), lost Maricopa by just 1,200 votes.

“It’s the lynchpin,” OH Predictive Insights pollster Noble said. “Historically, it’s incredibly hard to win without carrying the county.”

Arizona’s emergence as a swing state follows the path other Mountain West states like New Mexico, Colorado and Nevada have taken before, moving left as the population swells in major cities swamp smaller, rural areas that are losing residents. Only Texas, another state inching into competitive territory, has grown faster.

That growth is driven in almost equal parts by those under 45 and those who are older than 45, according to an analysis of U.S. Census data conducted by Jim Chang, Arizona’s official state demographer.

The population gains come at the same time as the Arizona Republican Party moves steadily to the right. A Republican Party once defined by the late Sen. John McCain is now dominated by Reps. Andy Biggs, Paul Gosar and Debbie Lesko, all members of the conservative House Freedom Caucus, and all ardent Trump backers.

Trump has refashioned the Republican Party so much in his image that former Sen. Jeff Flake (R), once seen as the hardest of hardliners and a thorn in the side of Republican leaders, is now backing Biden.

The confluence of a growing population, a conservative party shifting right and a personally divisive incumbent has put Maricopa County in the swing column for the first time in a generation.

“I’m the Jeff Flake Republican. That’s me, and I’d work on his campaign every day,” Cox said. “President Trump has done phenomenal things for my 401(k), but I also have children, and I don’t think he’s setting the right example for his country.”