"The American people have witnessed what is the greatest failure of any presidential administration in the history of our country," said Harris, a US senator from California and former prosecutor.
"And frankly, this administration has forfeited their right to reelection based on this," Harris said before a masked and limited audience at the University of Utah.
Saying Trump treated front-line health personnel as "sacrificial workers", Harris — pointing to Trump's own statements to journalist Bob Woodward — accused the White House of not moving quickly despite knowing the risks of Covid-19.
"The president said it was a hoax. They minimized the seriousness of it," Harris said.
After a raucous US presidential debate eight days ago between Trump and Biden, Pence and Harris took a more civil tone with no name-calling, but sharply disagreed on the reaction to the pandemic.
"I want the American people to know, from the very first day, President Donald Trump has put the health of America first," Mike Pence said, pointing to his ban on travel from China on January 31, a month after cases first emerged in Wuhan.
Referencing a controversy that tanked Joe Biden's first presidential campaign in 1988, Pence said the Democrats' Covid plan sounds "a little bit like plagiarism, which is something Joe Biden knows a little bit about".
In contrast to Donald Trump's firehose-like blasts on Biden and his family, Pence demonstrated calm and stability.