President Biden is cruising to the Democratic nomination. Former President Donald J. Trump could begin to wrap up his party’s nod within days. America’s response: This can’t be real. Even as both men stroll toward likely summer coronations and a fall rematch‚ an undercurrent of disbelief is coursing through the country. Many Republicans view Mr. Biden as so politically and physically weak that they think his party will replace him. Many Democrats can’t fathom that Mr. Trump...