My preference to lead this unity coalition would be Mike Bloomberg, because I think he has a campaign machine that is built to last and stand up to the pummeling from Trump, but he may not have enough grass-roots support in the party. If it’s not him, then the obvious choice would be Biden, whose campaign really has been re-energized. So far, Warren has not shown enough breadth of support.

(Disclosure: Bloomberg Philanthropies has donated to Planet Word, the museum my wife is building in Washington, to promote reading and literacy.)

If the Democrats can rally behind a consensus center-left candidate, he or she will be in a strong position to beat Trump for two reasons — one new and obvious, one deep, less obvious, but very powerful.

The first has to do with the coronavirus, which is reminding people why good government matters. So many people voted for Trump the last time because they wanted a disrupter who would shake things up. Well, he’s sure done that, running through multiple chiefs of staff and secretaries of defense and directors of national intelligence, not to mention four secretaries of homeland security and five national security advisers, not to mention reckless attempts to slash the budget of the Centers for Disease Control and Prevention, not to mention constantly denouncing the professional “deep state” civil servants whom we need now more than ever to protect our laws.

This epidemic is going to remind people how dangerous it is to have a disrupter with no ethics and no discipline. It is going to remind people that the G.O.P. laugh line — “I’m from the government and I’m here to help” — and other efforts to trash and weaken the federal state are not at all funny. It is going to remind people how important it is to have a president who appoints and values qualified people, not just loyalist hacks.

But the coronavirus and its aftermath also remind us how important it is to have not only a proven leader in the White House, but also one capable of pulling together a broad coalition of support. We will not defeat this virus as a house divided; we will not do anything important as a house divided.

Which is why I believe the hunger for a leader who can reunite the country is a stronger issue than experts realize. A Democratic candidate who can speak to that, inspire it and model it with his or her cabinet plans — by bringing together a broad range of moderate and progressive Democrats and moderate Republicans — will win.