Biden v Trump: 45 % if nothing changes drastically between now & November.
Biden v Trump: actually impossible to calculate because the odds of heart attack, stroke, senior moments, and death are actually not too low for men of their age in a given four-month period of high stress and full-time work, not to mention world events.
Anyone else v. Trump is the Democrats unite behind them NOW: 75%
Anyone else v. Trump in any remotely realistic scenario: 45%.