This fits into a line of attack that the Trump campaign is determined to press. No one believes that Joe Biden is a dangerous radical. But the president and his advisers charge that Mr. Biden will be a tool of leftists because he is beholden to them and too befuddled to stand up to them. The former vice president needs to show that this is not the case.

In 2018, educated suburban voters deserted moderate Republican incumbents and handed Democrats control of the House. One might have expected Donald Trump to respond, as Bill Clinton did after a Republican rout in 1994, by reaching out to voters who had registered their discontent. Mr. Trump has done the opposite, doubling down on energizing his shrinking base.

The president’s strategy has handed Mr. Biden an opportunity to realign American politics by cementing the suburbs into the Democratic coalition. He can do this while pledging to fight every form of discrimination against African-Americans and other minorities. He cannot do this if suburban voters doubt his resolve to protect domestic tranquility against disorder.

Despite the country’s dire circumstances, few Americans are in a revolutionary mood. They are tired of the tumult of the Trump years, and they yearn for a return to normal life. The Covid-19 pandemic and ensuing economic downturn have bred insecurity and a sense that events are spiraling out of control.