The Covid-19 crisis has upended life in America, including the presidential campaign. Both former Vice President Joe Biden and President Trump face new strategic challenges, and the country is watching.

As the U.S. rightly turned from primaries to quarantines, Mr. Biden was naturally sidelined. His first move to resist was to convert his home’s rec room into a studio, so he can appear on cable-TV shows. It wasn’t enough. His speech on Monday was vague and flimsy, marred by a malfunctioning teleprompter and a meandering discourse…