Pence, 61, and Harris, 55, were due to take the University of Utah stage in Salt Lake City, separated by plastic barriers for a 90-minute debate, a reminder of how the COVID-19 pandemic has influenced the race for the Nov. 3 election.

Now 77, Biden would be 78 upon taking office in January, which would make him the oldest US president in history. The Republican Trump is 74, making his COVID-19 all the more dangerous considering his age and underlying health issues. Trump already is the oldest president to seek re-election.

Trump announced on Friday that he had tested positive and was hospitalized for three nights and given treatments that continued with his return on Monday to a White House where the disease has spread. His doctor said on Wednesday that Trump had no COVID-19 symptoms for 24 hours.

Either of the vice presidential candidates would be a heartbeat away from assuming the presidency, depending on the election's outcome. How they handle themselves during the debate may be scrutinized even more closely than usual given Trump's health and the age of both presidential candidates.

The Pence-Harris debate is unlikely to match the chaos of the first debate last week between Trump and Biden, when Trump repeatedly interrupted Biden and the two traded insults. Pence, more even-tempered than Trump, and Harris, a former prosecutor known for sharp questioning during Senate hearings, are seen as polished communicators.

Hours before the debate, the Harris camp sought to ramp up expectations for Pence's performance.

"We know that Vice President Pence is a formidable debater, and that tonight is a challenge," said Democratic US Senator Cory Booker, a Harris ally.

Aides to Harris said that Pence's style would be more polished than Trump's more aggressive, off-the-cuff manner, but that his message would be the same.

Biden leads Trump in national opinion polls and has an advantage of 12 percentage points in the latest Reuters/Ipsos survey of likely voters. Polls show the race to be closer in some of the election battleground states that could determine the winner, although a Reuters/Ipsos poll on Wednesday showed Biden leading Trump in pivotal Florida.

Harris, who will be on the biggest stage of her political career, is a US senator from California picked by Biden in August as his running mate. The daughter of immigrants - her father from Jamaica and her mother from India - Harris is the first Black woman nominated by a major party for vice president as well as the first person of Asian descent.

Pence, a former conservative radio host who debated then-Democratic vice presidential nominee Tim Kaine in 2016, is a former US congressman and Indiana governor who has steadfastly defended Trump during his tumultuous presidency.

A Biden campaign official said Harris was prepared for Pence to attack her as too liberal, echoing Trump's assertion that the moderate Biden would advance a "radical left-wing" agenda if elected president, a charge Biden disputes.

The two campaigns have squabbled over installing barriers on stage between Pence and Harris to guard against coronavirus infection. On Wednesday, television images of the debate stage showed two plexiglass barriers, one adjoining each of their desks. Pence and Harris will be placed more than 12 feet (3.6 meters) apart on stage.

Harris had requested plexiglass shielding. Pence spokeswoman Katie Miller on Monday mocked Harris, saying if she "wants to use a fortress around herself, have at it." An aide to the vice president on Wednesday said Pence had agreed to the barriers to facilitate the debate.

Both Pence and Harris tested negative for the coronavirus on Tuesday. US government guidelines call for anyone exposed to a person with COVID-19 to quarantine for 14 days regardless of test results.

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