We’re watching Tuesday’s special election in Arizona closely, not because we expect the Democrats to stage an upset (the congressional district is solidly Republican), but because Republicans are showing concern over the outcome. Here’s what makes this House race interesting.

Results of the special election for Arizona’s Eighth Congressional District will be posted live starting around 11 p.m. Tuesday, Eastern. See results » Eighth Congressional District Phoenix Tucson

The district is deeply red.

Debbie Lesko, a former Republican state senator, is facing the Democrat Hiral Tipirneni, a doctor, in the race for the Phoenix-area seat that is reliably Republican.

After the district lines were redrawn in 2012, the district has voted Republican in the past three elections. In fact, Democrats did not even field a candidate in the last two elections. The district has also supported Republican presidential nominees by large margins.

Margin of victory in presidential elections +25 pct. pts. +20 Arizona’s Eighth District +15 +10 +5 2008 2012 2016 REPUBLICAN DEMOCRATIC U.S. +5 +10 +25 pct. pts. +20 Arizona’s Eighth District +15 +10 REP. 2008 2012 2016 DEM. U.S. +10 Sources: Federal Election Commission and DailyKos | Note: The margins for Arizona’s Eighth District for 2008 were calculated based on its current boundaries. In 2016, Hillary Clinton, a Democrat, won the popular vote, but Donald J. Trump, a Republican, won the electoral vote.

Donald J. Trump won the district by more than 20 percentage points in 2016. Four years earlier, Mitt Romney had won it by almost 25 points.

The Eighth District seat was vacated by Representative Trent Franks, a Republican who resigned after he was revealed to have offered $5 million to an aide in exchange for carrying his child.

But Republicans are worried enough that they spent a lot of money here.

To assure success, Republican leaders and groups have poured money into Ms. Lesko’s race, taking a variety of precautionary measures, like spending on ads and deploying robo-calls from Mr. Trump.

The Republican National Committee and the National Republican Congressional Committee have together spent more than $900,000 to boost Ms. Lesko.

Money spent to boost each candidate National Republican Congressional Committee Republican National Committee Other $527,211 $383,193 Lesko $249,295 Tipirneni RNC NRCC Other Lesko $527,211 $383,193 Tipirneni $249,295 Source: Federal Election Commission | Note: Numbers are as of April 23.

The House Democratic campaign arm and House Majority PAC — the best-funded House Democratic super PAC — have not supported Ms. Tipirneni to the same extent. That the national Democrats have not intervened indicates that they believe the predominantly white Eighth District, an area in and around Sun City and home to a sizable senior population, is out of reach.

It’s unlikely that the seat will flip Democratic, but a close margin is possible.

Two metrics offer otherwise-nervous Republican officials the most reassurance in a district where many of the ballots are cast early: Most people who have already voted are Republicans, and the median age of voters is 67.

Since early voting began on April 2, nearly half of the 150,000 Arizonans who have sent ballots are Republican, a sign that Ms. Lesko is most likely leading by a substantial margin.

Party affiliation of early ballots cast, as of April 20 Republican 48.6% Democratic 27.7 Independent 23.3 Republican 48.6% Democratic 27.7 Independent 23.3 Source: Arizona Secretary of State | Note: Numbers are cumulative from April 2 to 20.

Nearly a quarter of voters are independents, meaning Ms. Tipirneni would have to capture some Republican voters and win overwhelmingly among independents to claim victory.

Unlike with the House seat that Democrats won in Pennsylvania last month, where a Libertarian won 1,379 crucial votes, there is no third-party candidate on the ballot to make it easier for Ms. Tipirneni to prevail without a majority.

Most voters sending in ballots are nearing or enjoying retirement. Seniors are both the most conscientious voters and, in this district, largely conservative-leaning.

Median age of early voters 80 years 72 70 60 58 50 April 2 April 19 80 years 72 70 60 58 50 April 2 April 19 Source: Arizona Secretary of State. | Note: Dates reflect dates of tabulation and may not be when votes were cast. Data not available for some dates.

But in the final days of the race, the median age of voters has grown slightly younger — an indication that the contest has closed some and that less reliable Republicans are participating. The uptick in younger voters suggests the race has won attention in the broader, more politically mixed electorate.