The prospect of a cage match between a president for whom no subject is off-limits and a challenger who can be openly emotional is making some Biden advisers nervous. They see a fine line between Biden’s passion and empathy, which can appeal to voters, and the raw anger that sometimes gets him in trouble and could undercut his pitch as a calming alternative to a president who thrives on chaos.

“When you go at his family, he becomes hotter than hell, which is part of the thing I worry about,” said John Morgan, a Florida trial lawyer and major Biden donor. “I think what Biden has to be careful about is not letting his Irish temper blow when that happens.”…

Still, there are reasons for Democrats’ apprehension about their 77-year-old nominee, who last ran for office in 2012. In recent months, Biden has largely been cloistered away from hostile settings, and some supporters fret that he might be rusty when he confronts what could be some of the most vicious criticism he’s ever faced in person.

“It’s like training for a knife fight and somebody is getting an Uzi,” lamented one Democratic congressional aide.