Joe Biden said Donald Trump's move to nominate a new Supreme Court justice following the death of Ruth Bader Ginsberg amounted to an "abuse of power".
Asked what he thinks of leading contender Amy Coney Barrett, Biden said he doesn’t know her beyond what is reported in the press but that a lot was at stake in whoever is appointed.
The Democratic contender was mocked by the president after he put a "lid" on his appearances around 9am, meaning he wouldn't be emerging for any public campaigning.
In Biden's absence, Bernie Sanders took the stage to detail Trump's alleged efforts to "steal" the 2020 election, saying a vote in November wasn't about Biden but was about saving democracy.
It came after Trump refused to commit to a peaceful transfer of power "win, lose, or draw" in November's election, leaving Biden to ask "What country are we in?"
The threat of challenged election result was enough for a collective of almost 500 ex-national security officials to endorse Biden, including four-star generals and Republicans who served under Ronald Reagan and the Bushes, in a damning snub to the president.
Also responding to the shooting of two police officers in Louisville, Kentucky, on Wednesday night after new protests erupted over the ruling in the Breonna Taylor case, the Democratic nominee said: “Violence is never and can never be the answer.”