Life as the commander in chief is “in a sort of protected bubble,” says Alan Schroeder, a presidential debate historian. “It’s four years of people saying ‘Mr. President,'” a Democratic strategist put it.

Then comes the first debate of a re-election campaign — when the shock of personal confrontation with a hungry adversary has for decades led presidents to lose or perform poorly in that initial matchup, putting their chances of a second term in doubt.

It’s a trend President Joe Biden will…