LOS ANGELES — Christy Smith, a Democratic House candidate in suburban Los Angeles, had just finished a debate (over Zoom, of course) and was eager to point something out: First, President Trump had endorsed her Republican opponent, Mike Garcia. Then the president raised the specter of voting fraud, writing on Twitter: “Turn your Ballots in now and track them, watching for dishonesty. Report to Law Enforcement.”

Democrats saw this special election on May 12 as a referendum on Mr. Trump even before the coronavirus crisis brought his leadership front and center for many Americans. For Mr. Garcia, the president’s support means potentially more donors and a motivated, loyal base. For Ms. Smith, it’s a vulnerability to attack.

“The same week, we have a president who endorsed both my opponent and potentially using household disinfectants to treat a deadly pandemic,” Ms. Smith said from her home office in a recent interview. “I think that pretty well encapsulates the moment.”

The election is a microcosm of the country’s politics amid the health crisis: It is an early test of Mr. Trump’s sway in a race both he and his former rival, Hillary Clinton, have weighed in on. It is a battle over vote-by-mail in which doubts have been sown over the election’s integrity. And it is showing just how nasty politics can be, even under lockdown.

In the 2018 midterm elections, this Southern California district, the 25th Congressional, was one of the highest-profile victories for Democrats. But after just a year in office, Representative Katie Hill resigned after admitting to an affair with a staff member. Now, Ms. Smith and Mr. Garcia are locked in a bitter battle that will serve as an important early test for both parties ahead of the fall.

One key question is how much of a role Mr. Trump will play. Democrats believe that focusing on his leadership, particularly over the pandemic, will help them in a suburban district north of Los Angeles that Ms. Hill won by nine percentage points. But Republicans appear emboldened, counting on reliable conservatives to cast their ballots.

Each of the roughly 425,000 voters in the district was sent a ballot for the election — with return postage already paid. But there’s another unknowable: How much will it take to get voters to move those ballots from their kitchen counter to their mailbox at a time when many are consumed by worries about their health and finances?

If the choice is between “‘I’ve got to spend a little time thinking about who my congressional candidate is today’ or ‘I’ve got to figure out a way to get back online and apply one more time for my unemployment insurance that I haven’t gotten yet,’” the answer is obvious, Ms. Smith said. “People are going to take care of their families.”

“We get the challenge,” she added. “We understand how hard it is.”

Mr. Garcia, a defense contractor and political newcomer, has relied heavily on his biography — he was raised in the district, leaving after high school for the Navy, where he served as a pilot. Nearly all of his advertisements feature him standing in front of a plane, and his campaign logo is designed to resemble jet wings, with “fighter pilot” above his name on his website.

During a recent call with volunteers, Ms. Smith, a current member of the State Assembly with a long history in politics, laughingly questioned Mr. Garcia’s credentials.

“Did you guys know he’s a pilot?” she asked sarcastically. During the debate the day before, she said, she texted her team to point out that while he had “pictures of planes behind him,” her background was “constitutional law books.”

Mr. Garcia’s supporters seized on the remarks, saying they were evidence that she does not respect his military service. Mr. Garcia declined to comment for this article.

During the pandemic, the race has become increasingly vitriolic — with the campaigns unable to knock on doors, they have focused on blanketing television and social media with advertisements, many of them negative.

The race is also putting a sharp focus on the increasingly partisan debate over vote-by-mail, which Republicans have portrayed as ripe for fraud, though there is no evidence of widespread wrongdoing. Party officials have focused much of their ire on so-called ballot harvesting, the legal practice in which political organizers collect ballots from voters and drop them off at polling sites on their behalf.

Speaking to supporters in late January, Mr. Garcia said that he believed many votes were left uncounted and that the special election would “magnify or potentially open up the opportunity for more fraud than already existed,” both unfounded claims. Republicans in Congress, including Kevin McCarthy of California, the House minority leader, have repeatedly questioned the results of past elections in the state.

“The question is, how confident am I in the integrity of the election? Not confident at all,” Mr. Garcia said. “The bottom line is, I have very low confidence in a truly high-integrity election process.”

This month, Republican officials in California sent a letter to Gov. Gavin Newsom, a Democrat, demanding that he make such collections illegal amid stay-at-home orders. Last week, state party officials filed a lawsuit to try force such an action.

But while Ms. Smith said she was not allowing anyone from her campaign to collect ballots, Democrats say Mr. Garcia’s campaign appears to be setting up the kind of system his party has repeatedly condemned. Officials from his campaign have encouraged local churches to set up unofficial drop-off sites for ballots as recently as last month, according to an internal email provided by Democratic officials.

Alex Padilla, California’s secretary of state, has not issued any directive over collecting ballots, but has been unabashed in his response to accusations from Republicans that he has presided over elections rife with fraud — a claim he calls baseless.

“Voter fraud is nothing but a distraction and nothing but a pretext for suppressing the vote,” said Mr. Padilla, a Democrat. “It’s disingenuous at a time when we should be making it easier — not stifling rights.”

More than half of all voters in California have voted by mail for the last decade. Mr. Padilla and other election officials view the special election as a test run for November. Though state officials are still hammering out detailed plans, Mr. Padilla expects that voters statewide will automatically receive their ballots by mail.

Even then, local officials will still be expected to open in-person polling places, and they have begun to search for larger locations to allow for social distancing and for new volunteers to work the sites.

“The most important thing is to demonstrate that even during the Covid pandemic, our democracy is resilient, and that we can provide accessible and safe measures both now and especially for November,” Mr. Padilla said.

Early indications suggest voters are turning out in high numbers. Already, nearly 20 percent of voters in the district have cast their ballots, with 31 percent of registered Republicans and 19 percent of Democrats doing so, according to tracking data. Registered Democrats outnumber Republicans by nearly 30,000 voters in the district.

And Mr. Trump is hardly the only high-profile official paying attention to the race: Ms. Smith attracted endorsements from Senators Elizabeth Warren and Kamala Harris, and this week Ms. Hill’s new political action committee began a $200,000 advertising blitz to urge her former supporters to vote in the special election, targeting newly registered voters and those who cast a ballot in 2018 but had not consistently voted in congressional elections.

But many Democrats worry that Ms. Hill’s sudden resignation left the party vulnerable in the district, and strategists have privately reported that Ms. Hill’s high unfavorable ratings in the district have made it more difficult for Ms. Smith. In February, several California lawmakers met at the Democratic Congressional Campaign Committee headquarters and expressed concern that she was not raising the kind of money needed to win the district. Both Ms. Smith and Mr. Garcia have each raised over $2.2 million and have about $300,000 cash on hand, according to their most recent campaign finance reports. Last month, the Cook Political Report shifted the race from “lean Democratic” to a “tossup.”

During the 2018 midterm elections, activists from safe Democratic districts in Los Angeles routinely trekked an hour north to knock on doors in the middle-class district, which has long drawn families looking for more affordable suburban housing and has grown increasingly diverse — roughly 45 percent of the district is black, Latino or Asian.

Indivisible, the liberal group that helped flip several congressional districts in 2018 and has backed Ms. Smith, had so many volunteers for at-home phone banks in recent weeks that it had to create a waiting list and wrote postcards to send to every voter in the district.

“People are really motivated to defend what they won, and there’s definitely still a lot of energy among volunteers,” said Lucy Solomon, a national political director for the group. But she tempered her optimism. “It’s impossible to know how coronavirus is going to affect the outcome,” she said.

Though Mr. Padilla expects that the overwhelming majority of voters will choose to mail in their ballots, a few in-person voting options will be open on May 12, mostly to allow for same-day voter registration.

Still, he cautioned, if the results show a close race, a winner might not be clear for days, or even weeks. And no matter who wins, a rematch is expected in November, when the candidates will battle for the full congressional term.

Catie Edmondson contributed reporting from Washington.