The special election for Arizona's 8th Congressional District is finally here — and in a deep red district Republicans have never had to worry about before, the race is much closer than anyone could have predicted.

Earlier in the year, most predictive outlets would not have had the race for Arizona’s 8th Congressional District on their radars. The demographics and political landscape typically favor the Republican far too much for it to be competitive.

The seat was previously held by Republican Rep. Trent Franks. In December, Franks resigned in disgrace after it was revealed that he had offered a staffer $5 million to carry his child. Franks served seven terms, easily winning re-election each time.

But things have changed. Recent polling and a Democratic victory in a similarly Republican fortress in Pennsylvania’s 18th Congressional District in March has turned the political world upside down. The special election in Arizona, on April 24, is actually a competitive race.

On Tuesday, voters will choose between Democrat Hiral Tipirneni and Republican Debbie Lesko. Tipirneni is an emergency room physician who has been a public policy advocate for cancer and health care research. She has run very heavily on health care.

Lesko served as a state senator in Arizona’s legislature. She has run on several heavily pro-Trump positions like the border wall — her campaign page features a “build the wall” logo in the center — and preventing Medicaid expansion.

The district includes the northwestern suburbs of Phoenix, Peoria, and all of the locations around the northwest beltway.

It is a relatively wealthy district for the region, with a median income of $60,223. It is a relatively white population: 84 percent of its residents are white, and 17 percent are Hispanic. (This does not take ethnicity into account.)

It’s also an older district; only 19 percent of the voting eligible residents are under the age of 35. Nearly half of the voters in the district are senior citizens, and health care has been a major topic for Tipireni.

Health care is also a major issue for voters, who list it as one of their top four issues, according to a recent Emerson poll. Tipirneni has a substantial lead — 69 percent to 26 percent — among voters who say health care is their top issue.

It is easy to see why Republicans have historically fared so well in this district, which voted for Trump by a 21 percent margin, consistent with previous presidential elections. In November 2016, Franks won re-election by by nearly 29 percent. The Cook PVI value for the district is R+13.

The map below shows how the district voted in the 2016 election:

Map of Arizona's 8th Congressional District in 2016Benchmark Politics

So why is this competitive, and what does the Democrat have to do to even have a shot here?

Democrats are enjoying some of the highest congressional ballot numbers they have seen in a long time. This was enough to overcome a Cook PVI value of R+11 in Pennsylvania’s 18th Congressional District, where Democrat Connor Lamb won in an upset.

Arizona’s 8th is only slightly more Republican than that. Polling for the race has tightened, with one poll even showing Tipirneni in the lead. By all accounts, this race is not going to be a simple one for the Republican candidate like it has been in the past.

The Benchmark Model shows the race as Likely Republican, with a 6-point victory for the Republican candidate. (You can read about Benchmark’s methodology here.) Anything short of a double-digit victory by the Republican candidate would signal that going into 2018, hopes for a blue wave are alive.

Arizona 8th Congressional District statsBenchmark Politics

What to watch for as results come in:

The main thing to watch for is the Peoria-Sun City-Surprise region. It is in the northwest portion of the beltway, where Republican margins have been much smaller than the rest of the district.

Most of the population of the district lives here, and portions of this region have gone blue before. If precincts in Peoria proper go for Tipirneni, that means she has a shot at pulling the upset. If Tipireni wins precincts west of the beltway and further out — areas that are usually completely red — her chance of an upset is even stronger.

Be sure to follow @benchmarkpol on Twitter for live race results as they come in Tuesday night. Polls close at 10 p.m. Eastern, with results embargoed for an hour.