On paper, the state of Arizona is as red as its Cardinals. Republicans currently hold every statewide office and have won every presidential and Senate election for more than 20 years. But if you’re looking for the place that’s most likely to decide control of the U.S. Senate for the remainder of Donald Trump’s first term, Arizona should be at the top of your list.

This November, Democrats hope to take advantage of rapidly changing demographics and heightened party enthusiasm to break the Republican winning streak in the Grand Canyon state. The potential consequences of doing so are profound; by flipping one of Arizona’s Senate seats blue, Democrats could loosen Republicans’ grip on the Senate and make a bold statement about their electoral support in states won by President Trump in 2016.

On paper, the two Senate candidates are very similar. Both are fairly moderate, female Congresswomen who currently represent swing districts in the House. Kyrsten Sinema, the Democratic nominee, is a former social worker who went from growing up semi-homeless to serving three terms as a Representative from the state’s 9th district and a member of the House’s centrist ‘Blue Dog’ coalition. Martha McSally is a veteran who was the first female commander of an Air Force fighter squadron and who took Donald Rumsfeld to the Supreme Court for making her wear traditional Muslim clothing while on duty overseas (she won). In 2014, she beat a Democratic incumbent to win the race to represent the state’s 2nd district, and was easily reelected in 2016 to the same position. They are the strongest candidates that either party could have nominated, and experts rate the general election as a pure toss-up. As of this posting, FiveThirtyEight.com has Sinema leading with 50% to Sally’s 48%, whereas the New York Times’ live polling has McSally leading with 2 points, at 48%.

In many ways, the Arizona Senate race will forecast the political landscape for the remainder of President Trump’s first term. That the seat is open at all can be credited to the president, whose constant criticism of the retiring Republican incumbent, Jeff Flake, all but guaranteed he would lose the Republican primary if he ran for reelection. However, that move may backfire on the president, because open seats are almost always more competitive compared to those held by incumbents.

The candidates’ strategies reflect two different philosophies on how to succeed in the Trump Era. Sinema is running what the New York Times called “one of the most moderate-sounding and cautious Senate campaigns this year.” She largely avoids the media and discussing President Trump. McSally, on the other hand, is running as a Trump acolyte: She is running ads comparing their positions on immigration and has voted for every one of his major bills in Congress. That position helped her win a competitive Republican primary over more conservative opponents, but it may hurt her in the general election given the state is rapidly becoming less red.

The candidates’ differences are also reflected in their rhetoric on TV and on the campaign trail. Sinema has largely refrained from attacking the President or her opponent, instead relying on her compelling personal story: a journey from homelessness to a Ph.D. That story has driven her rise through state politics and into the House, but now the story itself is under fierce attack, with both her opponent and multiple news organizations questioning some details. Sinema has also been attacked in TV ads for being anti-troop and pro-human-trafficking, over her protests against the Iraq war and a vote she took in the state legislature. Analysis by CNN found that both ads were factually inaccurate.

Given the state’s Republican lean, this seat should not be competitive. Romney won it by 10 points even as he lost by a wide margin nationally, and Republicans won the last two gubernatorial elections by 12 points each. But the state is rapidly shifting towards the center: Trump only won it by 3.5 points in 2016. For an explanation of that shift, there is no better place to go than Maricopa County, home to most of the state’s golf courses and voters.

It’s hard to find a place that better embodies the term ‘sun belt’ than Maricopa County. . Its voters have provided the necessary margins to ensure Republican victories in every Presidential and Senate election for more than 20 years. The county also elected and re-elected Sheriff Joe Arpaio; perhaps the most famously anti-immigrant officeholder in the country.

But Maricopa County, like the rest of Arizona, is changing. Its share of white residents, once overwhelming, is dropping, from 78% to 73% in 2000-2010. At the same time, many of the suburban and highly-educated of those white residents are also trending less Republican. Mitt Romney won the county by 10 points, even while losing badly on the national scale, while Donald Trump only won the county by 3.5 points, mirroring the statewide margins almost exactly. Arpaio, for one, was not immune from the state’s leftward shift: he fell to a Democratic challenger in 2016 while seeking a 7th term.

Along with Arpaio's demise, there are two other factors in Maricopa that could spell trouble for McSally. The first was April’s special congressional election in Arizona’s 8th Congressional district, contained entirely within Maricopa County. In 2016, the 8th voted for Trump by 21 points, but in April Debbie Lesko, the Republican nominee, won by less than five points, indicating a dearth of enthusiasm among Republican base voters in Maricopa County. If there is a similar drop off among Republican base voters this November, McSally will lose--and not by a little. The other problem for McSally as far as Maricopa County is concerned is that Sinema’s district takes up a portion of the county’s swing turf, meaning she already has a history of winning tough elections on the same turf that will decide the winner this November.

Ultimately, the candidate that replaces Jeff Flake will depend on whichever party base is more motivated, and whether the Republican-leaning independents--the state’s primary swing voters-- prefer McSally’s embrace of Trump or Sinema’s centrist rhetoric. One thing is for certain: the results of this race will have implications for 2020 and beyond. For starters, if Democrats don’t flip this seat, their odds of capturing the Senate majority are next to nil. If Sinema wins, all eyes will turn to 2020, when Arizona’s other Senate seat and the state’s 11 electoral votes are up for grabs. If, on election night, you want a bellwether for the midterms and a preview for 2020, the Grand Canyon State would be a good place to look.

An Election in the Land of Golf and Sun: the 2018 Senate Race in Arizona