Maybe Buttigieg is unaware of the growing consensus among mainstream economists that the deficit hysteria of seven or eight years ago was greatly overblown. Last year the former top economists in the Obama administration published an article titled “Who’s Afraid of Budget Deficits?” which concluded, “It’s time for Washington to put away its debt obsession and focus on bigger things.”

And where Sanders is playing right into one disreputable Republican political strategy, Buttigieg is playing into another: the strategy of hobbling the economy with fiscal austerity when a Democrat occupies the White House, then borrowing freely as soon as the G.O.P. regains power. If Democrats win, they should pursue a progressive agenda, not waste political capital cleaning up the G.O.P.’s mess.

Again, if Buttigieg somehow becomes the nominee, the party should back him without reservation. Whatever he may say about deficits, he wouldn’t do what Republicans do: use debt fears as an excuse to slash social programs.

So who will the Democrats nominate? Your guess is as good as mine. What’s really important, however, is that the party stays focused on its strengths and Trump’s weaknesses.

For the fact is that all of the Democrats who would be president, from Bloomberg to Bernie, are at least moderately progressive; they all want to maintain and expand the social safety net, while raising taxes on the wealthy. And all the polling evidence says that America is basically a center-left nation — which is why Trump promised to raise taxes on the rich and protect major social programs during the 2016 campaign.

But he was lying, and at this point everyone with an open mind knows it. So Democrats have a perfect opportunity to portray themselves, truthfully, as the defenders of Social Security, Medicare, Medicaid and the now-popular Affordable Care Act against Republicans who are more or less nakedly favoring the interests of plutocrats over those of working families.

This opportunity will, however, be squandered if the Democratic nominee, whoever he or she is, turns the election into a referendum on either single-payer health care or deficit reduction, neither of which is an especially popular position. Things will be even worse if the Democrats themselves degenerate into squabbles over either ideological purity or fiscal probity.

The point is that whoever gets the nomination, Democrats need to build as broad a coalition as possible. Otherwise they’ll be handing the election to Trump — and that would be a tragedy for the party, the nation and the world.

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