PHOENIX (AP) — When Katherine Rutigliano and her husband moved away from San Francisco in 2013, they figured they would never meet a fellow Democrat again.
But housing was affordable around Phoenix. Everything would be easier for them and their kids in the suburbs — everything, that is, except talking politics with neighbors.
Then came an unexpected visitor at the door. It was a Democratic volunteer rounding up votes ahead of the 2018 Senate election. Rutigliano invited her in and inspected the map on her iPad. She was elated to see all the flashing lights that marked where Democrats lived in her stucco neighborhood on the northern edge of Phoenix.
These San Francisco transplants were not alone.
“It was like Christmas,” said Rutigliano, 37, a mother of three and trained chef who is now sending out mailers for local Democrats.
Rutigliano didn’t realize it, but she had moved her family to what is now the front lines in American politics. Once firmly in Republican control, suburbs like hers are increasingly politically divided — a rare common ground shared by Republicans and Democrats.
As such, they are poised to decide not just who wins the White House this year but also who controls the Senate and the contours of the debate over guns, immigration, work, schools, housing and health care for years to come.
The reasons for the shift are many. Suburbs have grown more racially diverse, more educated, more economically prosperous and more liberal — all factors making them more likely to vote Democratic. But demographers and political scientists are just as likely to point to another trend: density. Suburbs have grown more crowded, looking more and more like cities and voting like them, too.
For decades, an area’s population per square mile has been a reliable indicator of its political tilt. Denser areas vote Democratic, less dense areas vote Republican. The correlation between density and voting has been getting stronger, as people began to sort themselves by ethnicity, education, personality, income and lifestyle.
The pattern is so reliable it can be quantified, averaged and applied to most American cities. At around 800 households per square mile, the blue of Democratic areas starts to bleed into red Republican neighborhoods.
A purple ring — call it the flip zone — emerges through the suburbs.
But the midterm elections of 2018 showed that the flip zone has moved in the era of President Donald Trump, with dramatic consequences. When Democrats across the country penetrated deeper into the suburbs, finding voters farther away from the city, they flipped a net 39 House districts and won a majority of the chamber.
An Associated Press analysis of recent election results and density shows Democrats in Arizona moved the flip zone 2 miles deeper into the suburbs from 2016 to 2018, reaching right to the northern edge of Interstate 101 in Phoenix, into areas filled with cul-de-sacs of homes and backyards large enough for swimming pools. The shift helped them win a Senate seat for the first time in 24 years.
Many political scientists think the trend toward political segregation has put the Democratic Party at a disadvantage. Its voters are more concentrated in cities. Republicans are dispersed across larger areas, making it easier for that party to draw favorable districts and win a majority of legislative seats even if it loses the total vote count.
The geographic divide has also had a real impact on policy and politics. The needs of cities and farm towns are often perceived as being in conflict — a tug of war between Republican and Democratic voters over resources.
Jonathan Rodden, a Stanford University political scientist and author of the 2019 book “Why Cities Lose,” said this political divide on density has eroded the shared responsibility among elected leaders. Instead, they think of themselves as representing different voter groups and that gives them less incentive to work together on issues like the pandemic.
“Municipal officials can blame state and federal officials, who in turn blame lower-level officials,” Rodden said.
It’s not surprising that Trump and Democrat Joe Biden have been tussling over suburbia — a majority of the electorate lives there.
Trump has suggested that efforts to racially integrate the suburbs would destroy those communities with crime and poverty, despite clear data showing that many suburbs are increasingly diverse. At the first presidential debate, he accused Biden of wanting to kill off the suburbs.
“Our suburbs would be gone,” Trump said.
“He wouldn’t know a suburb unless he took a wrong turn,” Biden responded.
But the suburbs are not uniform. They vary from city to city and so do the flip zones, AP’s analysis shows.
— In Dallas, the purple ring through the suburbs in 2016 was 18.7 miles out from city hall, at an average of 714 households per square mile. The border runs close to AT&T Stadium in Arlington, where the Dallas Cowboys play. Arlington is a so-called boomburb that morphed through new construction from a suburb into a city of 400,000.
— In Atlanta, the flip zone was nearly 24 miles out, at 434 households per square mile. It stretched out to diverse suburbs such as Kennesaw, where Black and Latino residents have nearly doubled their share of the population in the last two decades.
— In reliably Democratic Boston, Chicago and Seattle, one must drive out more than 40 miles, to what is essentially farmland, to find the flip zone.
Now the suburbs are the places delivering a referendum on Trump. And neatly manicured neighborhoods conceal a more complicated political biosphere.
Republican Michael Nudo, 27, grew up in the Phoenix flip zone — when it was more securely Republican territory. During his freshman year in high school, his family lost their house to foreclosure as millions of other Americans did during the Great Recession. Then their rental house was foreclosed on, and they had to move again.
The experience instilled in him a conservative belief that the government, like families, must be financially responsible.
Now Nudo sees that housing crash as the beginning of another wave of change in his hometown — “a huge turnover.” As the economy recovered, big companies relocated workers from around the country and Democratic voters arrived.
“You can walk across the street and be in a whole other community, whole other city,” he said. “But they’re your neighbors.”