Greenwich, Connecticut - During a Monday fundraiser in Connecticut, Joe Biden blasted Donald Trump as a "convicted felon" seeking the US presidency for the first time in history.

Biden (81) was using the description for the first time since Trump was convicted by a New York jury of covering up hush money payments to a porn star in a historic verdict.

"Folks, the campaign entered uncharted territory," Biden told donors in Greenwich. "For the first time in American history, a former president that is a convicted felon is now seeking the office of the presidency."

Biden said Trump's attacks on the justice system, with the Republican former president making unsupported allegations that the case was rigged, were more dangerous still.

Donald Trump Trump ally says transition team ignoring rules is part of "hostile takeover"

"As disturbing as that is, more damaging is the all-out assault Donald Trump is making on the American system of justice," Biden said.

The Democratic incumbent said Trump, whom he beat in the 2020 election, would pose a greater threat if he wins a second term.

"Something snapped in this guy for real" after 2020, said Biden. "It's literally driving him crazy."

Biden also criticized Trump for suggesting that sending him to prison could prove a "breaking point" for his supporters – a warning that will fuel concerns of political violence around the presidential election on November 5.

"He says if he loses there will be a bloodbath in America. What kind of man is this?" Biden told supporters.

Biden's only previous comment on the Trump verdict, at the White House on Friday, was limited to saying no one was above the law – but without calling him a felon.

The Democrat's campaign has been treading a fine line on how far to go in using Trump's conviction to attack him, with some commentators urging Biden to keep a presidential distance and focus on election policies instead.