‘Donald Trump is not responsible for COVID-19, but he is responsible for our slow, uncoordinated response,” Joe Biden tweeted Friday. “Experts say that if we had acted two weeks earlier, more lives could have been saved. Trump failed to take swift action—and we’re paying the price.”

The Trump-acted-too-late story line would be more convincing if Mr. Biden and other Democrats had called for bolder action early in the crisis, but they didn’t. Democratic candidates held five televised debates, lasting nearly 11 hours from Jan....