For years, Arizona was to Democrats what Lucy's football was to Charlie Brown. Despite candidates from Barack Obama to Hillary Clinton investing in the state, no Democratic presidential candidate has carried it since Bill Clinton in 1996. In fact, no Democrat won a statewide election in Arizona on any level after 2008 until 2018, despite numerous close calls.

But Arizona is changing.

In the 2008 and 2012 presidential races, the state was 16 points and 13 points more Republican-leaning than the country as a whole, respectively. But in 2016, President Trump won Arizona by only 4 points, making the state just 6 points more Republican-leaning than the nation. And in 2018, four Democratic candidates broke through and won statewide, including Sen. Kyrsten Sinema.

Now, in 2020, Joe Biden looks like he has a chance to actually win Arizona's 11 electoral votes. As of June 29, Biden led Trump by 4.7 points in our Arizona polling average. And it looks like Democrats could flip another Senate seat here too, as Democrat Mark Kelly leads Republican Sen. Martha McSally by double digits in numerous polls.

Much of that is because of an extremely pro-Democratic national environment; according to our polling averages, Arizona is still a bit more Republican-leaning than the nation as a whole (4.6 points more Republican-leaning, to be precise). But if the final election results were to exactly match our current polling averages, it would still represent the third consecutive presidential election where Arizona has moved left.

So what's driving this shift?

Part of it is the same reason people have been predicting a blue Arizona for years: Latino voters. Along with the state's small Black and Native American populations, Latinos constitute the Democratic base in Arizona. In 2016, a precinct-level regression analysis estimated that Clinton won more than 80 percent of the Latino vote in Arizona. And according to an analysis from the Center for American Progress, the share of eligible Latinos who voted also increased from 37 percent in 2012 to 42 percent in 2016.

And Arizona's Latino population is swelling. The state has gone from 25 percent to 31 percent Latino since 2000. That said, the white population share in Arizona is still much higher (currently 55 percent). And many of Arizona's Latinos are ineligible to vote: Among U.S. citizens who are 18 and older, white people are 65 percent of the population and Hispanic or Latino people only 23 percent. Worst of all for Democrats, low turnout rates mean Latinos constitute an even smaller share of the actual electorate: According to the CAP analysis, 2016 voters in Arizona were 73 percent white and only 17 percent Latino.

So this trend alone doesn't explain Arizona's sudden competitiveness, even though the Latino share of the electorate is slowly but surely increasing (it rose by 2 points from 2012 to 2016). The bigger factor at play is one that is not unique to Arizona, either: The movement of suburban voters from Republicans to Democrats since the 2016 election.

Politically, culturally and economically, Arizona is dominated by Maricopa County, which covers Phoenix and its sprawling metropolitan area. In the last several elections, Maricopa has consistently accounted for about 60 percent of the votes cast in Arizona, which means that the candidate who wins Maricopa usually wins Arizona.

And for years, it was a Republican. Unlike in many states, the most Democratic parts of Arizona actually lay outside its biggest metropolis: Apache County (which includes much of the Navajo Nation and is 75 percent Native American), Coconino County (home of Flagstaff), Pima County (home of Tucson) and Santa Cruz County (a poor, rural county that is 83 percent Latino). As a result, Democrats consistently did better in the rest of Arizona than they did in Maricopa - where most of the votes were.

Until 2016.

Clinton lost Maricopa County by just 3 points (48 percent to 45 percent), a drastic improvement from the last four Democratic presidential candidates. And, notably, she became the first Democratic presidential candidate since at least 1960 to do better in Maricopa than she did in the rest of the state (where she lost by 5 points). Sinema made even more inroads in 2018: She won Maricopa County 51 percent to 47 percent while losing the rest of the state 49 percent to 48 percent. In other words, Maricopa County was the reason Arizona voted Democratic in 2018.

Because of its size, Maricopa is home to all sorts of areas, from heavily Latino and Black South Phoenix to historically Mormon Mesa to the college town of Tempe to retirement communities like Sun City. But the county's transformation has been led by upper-class suburban enclaves like Ahwatukee, Scottsdale and Paradise Valley. According to data from Daily Kos Elections, the state legislative districts where Clinton improved on Obama's performance the most also tended to be highly college-educated and have high median incomes.

Basically, Arizona's urban vs. rural divide is deepening, just like the rest of the nation's. But because Arizona is one of the most urbanized states in the country, that's a good trade for Democrats. In fact, according to an analysis based on FiveThirtyEight's urbanization index, if Arizona's density had been the only factor in how it voted, it would have voted for Clinton by 6 points.

And that may happen for Biden this year. Since March, Biden has held a small but consistent lead over Trump in polls there. Most recently, a poll by Siena College/The New York Times Upshot - one of the best pollsters in the business - gave him a 7-point lead among registered voters (although this will probably shrink among likely voters). But for now, it looks like the Democratic Party's newfound suburban strength, combined with the gradual growth of Arizona's Latino population, is finally putting the Grand Canyon State in play.

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