WASHINGTON—Former Vice President Joe Biden and Vermont Sen. Bernie Sanders will go head-to-head for the first time in a prime-time debate, as a global pandemic has upended not only the contest for the Democratic presidential nomination but the everyday lives of most Americans.

The debate Sunday night comes at a moment unlike any in recent memory. States are postponing primaries, governors are banning mass gatherings, schools are closing and sports leagues are canceling major events. Many Americans are largely confining themselves…

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