Joe Biden’s infirmities and their contributions to the president’s maladroit performance have imposed a paradox on the country. The Democratic Party has been reduced to making the negative case for Biden — not that he is a particularly adept president or that his presence in the Oval Office is desirable in itself, but that he is a better steward of the executive branch than Donald Trump. That wouldn’t be a remarkable strategy for an unpopular incumbent save the fact that the incumbent and...