The Trump years have been rough on Democrats’ sensibilities, and their thinking has become increasingly addled as a result. The party has worked tirelessly to create an issue worthy of impeaching the president—Russia collusion, obstruction of justice, Stormy Daniels, tax returns. This week Democrats jettisoned all that in favor of the only issue that implicates their own front-runner for the nomination. Genius.

The one person who has been as much in the news this week as Donald Trump is former Vice President Joe Biden. It’s...