By Tarini Parti, Andrew Restuccia, Wall street Journal–

Sen. Kamala Harris sought to make the election a referendum on the Trump administration’s handling of the coronavirus pandemic in a debate Wednesday that took on new importance following the president’s Covid-19 diagnosis, while Vice President Mike Pence said the Democratic presidential ticket’s policies were too liberal and would damage the economy.

Over the course of a sometimes-testy 90 minute debate Wednesday in Salt Lake City, Mr. Pence defended the president’s response to the pandemic and touted the economy and trade deals under President Trump’s leadership.

The California Democrat tried to hold the Trump administration responsible for failing to stem the spread of the virus, criticizing Mr. Pence, who leads the White House’s task force on the coronavirus, for the climbing death toll.

“The American people have witnessed what is the greatest failure of any presidential administration in the history of our country,” Ms. Harris said at the start of the debate, which also covered the Supreme Court, taxes, fracking and the duties of the vice president.

Ms. Harris said Mr. Trump has “forfeited his right to reelection” as a result of his handling of the pandemic. The vice president responded that Mr. Trump “has put the health of the American people first.”

Mr. Pence added that the administration had already implemented some of the plans Democratic nominee Joe Biden has proposed to tackle the virus, asserting that the Democratic plan “looks a little bit like plagiarism”—an allusion to Mr. Biden having copied remarks during his 1988 presidential campaign from a British politician.

The event had fewer fireworks than the recent presidential debate. The candidates, who were sitting at desks instead of standing, interrupted each other less often and their tones were more measured in comparison.

But there was still tension between the candidates. “Mr. Vice President I’m speaking, I’m speaking,” Ms. Harris said several times throughout the night. Mr. Pence similarly urged Ms. Harris to let him speak when she interrupted, and both candidates fought for additional time to speak.

The debate comes less than a month before the presidential election as the candidates competed for an increasingly narrow slice of undecided voters. Polls show many Americans have already made up their minds and millions have already voted.

Mr. Pence and Ms. Harris, a California Democrat, were separated by plexiglass and sat 12 feet apart during the event, a reminder of the coronavirus pandemic that continues to spread across the country and through the halls of the White House. Debate attendees received mandatory coronavirus tests ahead of the event, donning a green wristband to signify that they had tested negative. They also were required to wear masks. An audience of roughly 90 people watched the debate from the University of Utah’s Kingsbury Hall.

Turning to pocketbook issues, the vice president pledged that 2021 would be “the biggest economic year in the history of this country,” and accused Mr. Biden of planning to undo the economic achievements of the Trump administration by raising taxes. “The American economy, the American comeback is on the ballot,” the vice president said.

Ms. Harris countered that Mr. Biden wouldn’t raise taxes for anyone making under $400,000 a year, and she said the president is taking credit for the economic recovery started during the Obama administration, when Mr. Biden was vice president.

However, the coronavirus pandemic loomed large as a theme throughout the debate.

The vice president defended the White House’s decision to host a Rose Garden ceremony for Supreme Court nominee Amy Coney Barrett late last month. Several attendees have since contracted the coronavirus.

“President Trump and I trust the American people to make choices in the best interest of their health,” he said.

Ms. Harris said she would only take a Covid-19 vaccine approved under the Trump administration if the government’s public health experts vouched for it. “If Donald Trump tells us that we should take it, I’m not taking it,” she said. The vice president urged Ms. Harris not to raise doubts about the safety of a vaccine. “Stop playing politics with people’s lives,” he said.

The U.S. leads the world in both confirmed Covid-19 cases and deaths. With more than 7.5 million confirmed infections since the beginning of the pandemic, the U.S. accounts for nearly a fifth of the 36 million cases reported globally. More than 211,00 Americans have died.

The president’s illness revived concerns about the advanced age of both Mr. Trump, 74, the oldest man to assume the presidency, and the Democratic nominee Mr. Biden, 77, who would take that mantle if he is elected. If the president becomes unable to perform his duties, power can be transferred to the vice president, who is first in the line of succession.

Mr. Pence and Ms. Harris didn’t answer a question about whether they have spoken to their running mates about possible safeguards and procedures in the event that thy become incapacitated and can no longer lead the country.

Later, Mr. Pence didn’t answer a question about how the Trump administration would protect pre-existing conditions if the Affordable Care Act is struck down by the Supreme Court. The Obama-era health law requires that insurers cover people with pre-existing conditions. But the administration is supporting a Republican-led lawsuit that could invalidate the health law.

The vice president pressed Ms. Harris on whether Mr. Biden would push to add more justices to the Supreme Court if he’s elected. Ms. Harris didn’t directly say whether Mr. Biden would endorse that proposal, which some Democrats have advocated for if Judge Barrett is confirmed to the high court. Mr. Biden refused to answer a similar question in the first presidential debate.

The vice president repeatedly defended the president in his responses, reiterating Mr. Trump’s response to a report in the New York Times that stated the president paid $750 a year in federal income taxes in 2016 and 2017. “The President said those public reports are not accurate,” he said when Ms. Harris brought it up as an example of the lack of transparency from the administration. Mr. Trump hasn’t released his tax returns.

The vice president brought up Ms. Harris’s comment from the Democratic primary that she would ban fracking as evidence that a potential Biden administration would cost jobs in the heartland.

Although Ms. Harris called for the ban during a forum last year and co-sponsored the Green New Deal in the Senate, the plan her presidential campaign released didn’t include such a ban, just more disclosure of the chemicals used in fracking. Ms. Harris is now backing Mr. Biden’s plan to address climate change that includes a banning on fracking only on federal lands. “First of all, I will repeat, and the American people know, that Joe Biden will not ban fracking. That is a fact,” Ms. Harris said.

Ms. Harris painted the president’s trade policy as a failure. “You lost that trade war—you lost it,” she said. Mr. Pence shot back: “Lost the trade war with China? Joe Biden never fought it.”

The California senator with most Democrats and opposed Mr. Trump’s renegotiated trade agreement with Canada and Mexico because she didn’t view it as aggressive enough on climate change.

The candidates clashed on a range of foreign policy issues, from Russia to Americans being held hostage abroad. When Ms. Harris referenced a recent news report alleging that the president had called Americans who died in war “losers” and “suckers,” Mr. Pence raised his voice. “The slanders against President Donald Trump regarding the men and women of our armed forces are absurd,” he said.

During the debate, Mr. Trump praised the vice president’s performance. “Mike Pence is doing GREAT! She is a gaffe machine,” he wrote on Twitter.

Mr. Trump returned to the White House on Monday after spending several days at Walter Reed National Military Medical Center over his Covid-19 diagnosis. The president’s physician said earlier Wednesday that Mr. Trump had gone 24 hours without any coronavirus symptoms.

More than a dozen administration officials and others close to the president have tested positive for the virus and several more have been forced to quarantine. Senior White House adviser Stephen Miller was the latest to test positive for coronavirus on Tuesday.

A Wall Street Journal/NBC News poll released Sunday found that Mr. Biden leads the president, 53% to 39%, among registered voters. The poll, which was conducted after the first debate but before Mr. Trump’s diagnosis, shows Mr. Biden expanding his lead with women voters and the president losing support among white, working-class voters and other groups. Several surveys of states Mr. Trump won in 2016, including Pennsylvania, in recent days also show Mr. Biden widening his margin.

Although two more debates between Mr. Trump and Mr. Biden are scheduled, it remains unclear if they will take place. The president said Tuesday he wants to participate in a second debate against Mr. Biden scheduled for Oct. 15, which would be two weeks after the president first tested positive.

The Centers for Disease Control and Prevention’s guidelines say people who have contracted the virus can stop isolating after 10 days have passed since the onset of symptoms, as long as they haven’t had a fever in 24 hours and their other symptoms are improving.

Mr. Biden told reporters Tuesday he was looking forward to debating the president, but he said the debate should not take place if Mr. Trump still has Covid-19. “I just hope all the protocols are followed,” he said.

Write to Tarini Parti at Tarini.Parti@wsj.com and Andrew Restuccia at Andrew.Restuccia@wsj.com