 His argument rests largely on the fact that he beat Trump in 2020. That is no longer a sufficient rationale for why Mr. Biden should be the Democratic nominee this year. At Thursday's debate, the president needed to convince the American public he was equal to the formidable demands of the office he is seeking to hold for another term. Voters, however, cannot be expected to ignore what was instead plain to see. Mr. Biden is not the man he was four years ago. As it stands, the president is engaged in a reckless gamble.