Arizona’s eighth congressional district in Phoenix’s western suburbs is perhaps best known as a winter retreat for midwestern snowbirds – a sunny destination for retirees where cars share the road with golf carts. What the conservative district is not known for is electoral suspense.

Yet on Tuesday the Republican stronghold will play host to a special election that is being closely watched as a potential bellwether of Democratic enthusiasm before the midterm elections.

Arizona has emerged as a centerpiece in the battle for control of Congress. In the Senate, Republicans are defending the seat vacated by retiring senator Jeff Flake. Democrats are bullish about their chances. The state’s other senator, John McCain, has been absent from Washington since December: he is being treated for an aggressive form of brain cancer.

In the eighth district, Republican Debbie Lesko, a state senator, is the strong favorite to replace Trent Franks, the veteran congressman who resigned in December amid allegations of sexual misconduct. But Democrat Hiral Tipirneni, a political newcomer and former emergency room physician, has mounted a competitive challenge, with polls showing a tightening race.

Few expect an upset. Early voting returns favor Republicans and recent polling shows Lesko winning by anywhere from 10 and six points to a virtual tie. Donald Trump won the district by 21 points in 2016.

Republican Debbie Lesko, a state senator, is the strong favorite to replace Trent Franks. Photograph: Matt York/AP

Analysts are, however, monitoring the race as a test of whether Democrats can capitalize on Trump’s unpopularity in a year when the president’s party historically loses seats. A narrow victory for Lesko in a district where Democrats have not fielded a candidate since 2012 could “set off alarm bells” for Republicans, said Geoffrey Skelley, a political analyst at the University of Virginia’s Center for Politics.

“The question is: does the Republican win by single or double digits?” he said, adding that a spread of less than 10 points would be “notable”.

The race has drawn parallels to last month’s special election in western Pennsylvania, where the Democrat Conor Lamb won a shock victory in a traditionally conservative district that Trump carried by 20 points. But political strategists caution against comparisons between the two districts. One key difference is that Republicans hold a 17-point voter registration advantage in the Arizona district. The Pennsylvania district had more registered Democrats.

“District eight is a hardcore white Republican district that is about as red as red can be,” said Chuck Coughlin, a GOP consultant in the state. “The math just doesn’t add up.”

Voting began several weeks ago and according to early figures tracked by the Arizona secretary of state’s office, nearly half of the 150,000 ballots returned have been from Republicans. By comparison, 28% of ballots received were from Democrats. Nearly a quarter were from independents.

The numbers only reflect the voters’ party identification, not how they voted. But to overcome Lesko, Tipirneni would need to win some disaffected Republicans and dominate independents.

Despite the long odds, Democrats in the district are energized in a way local officials say they have not seen in recent memory. This spring the state party opened a field office in the West Valley – a first as far as anyone there can recall and a sign Democrats are committed to competing in even the reddest corners.

In an interview with the Guardian last month, Tipirneni said Democratic victories in Pennsylvania and Alabama showed that “conventional wisdom doesn’t apply”. She added: “There’s a lot of reason to think that Arizona can absolutely be next.”

Republicans are not leaving anything to chance. The Republican National Committee, National Republican Campaign Committee and the Congressional Leadership Fund have collectively spent nearly $1m to defend the seat – a sign, Democrats say, of their concern. National Democrats have stayed out of the race, which Republicans say indicates the district is out of reach.

Lesko has aligned herself with Trump, especially on immigration, supporting his desire to build a wall along the southern border. In a debate, Lesko sought to cast Tipirneni as too liberal and attacked the Democrat for opposing the wall and the Republican tax plan. On Tuesday afternoon, Trump urged Arizonans to vote for Lesko.

Arizona, please get out today and vote @DebbieLesko for Congress in #AZ08. Strong on Border, Immigration and Crime. Great on the Military. Time is ticking down - get out and VOTE today. We need Debbie in Congress! — Donald J. Trump (@realDonaldTrump) April 24, 2018

Tipirneni countered Lesko’s attacks with an ad that called her opponent “everything you hate about politics”. During the campaign, Tipirneni focused on healthcare and promised to protect social security and Medicare, federal programs popular with older voters.

On Monday night, Tipirneni held a final campaign rally with the former Arizona congresswoman Gabby Giffords and her husband, Mark Kelly, in Surprise. Giffords urged voters to find the “courage” to send Tipirneni to Washington.

Republicans warned that a narrow victory could send the wrong message. “She has got to crush this,” congresswoman Martha McSally, a Republican running for the US Senate, said at a campaign event for Lesko on Saturday. “We’ve got to make sure that this district knows, this state knows and this country knows: CD 8 is red. Arizona is red.”