Joe Biden said his dead son Beau "wasn't a loser or a sucker", saying the president doesn't understand honour, duty and what it means to serve something bigger.
It came at a campaign rally in Michigan, in which the former VP said the president "failed to do his job on purpose" after revelations in Bob Woodward's new book that Trump admitted to playing down the dangers of the coronavirus.
Mr Biden warned Trump about fastracking a potential coronavirus vaccine, urging the president to respect “the independent judgement” of government officials following promises to deliver a treatment before 3 November.
The Michigan campaign stop was ostensibly to announce job creation initiatives and his Made in America tax policy, which takes a 10 percent hit from companies that offshore production while giving 10 percent to companies that invest in American jobs.
But he was really there because his campaign believes they can win Michigan's white working class voters, telling the crowd he would be the most union-friendly president in US history.
More new polls released on Wednesday, meanwhile, showed the two candidates tied in the crucial swing state of Florida. And if Trump loses Florida, a pollster from Marist College Institute for Public Opinion said it's "game over".
The Biden campaign out raised the Trump campaign in August, raking in $364.5m for the month compared to their opponent's $210m. They spent a big chunk of that in the past week alone, with $17m on TV and digital across nine swing states. In total, Biden has outspent Trump $310m to $295m.