A barometer, said a wit, is an ingenious instrument that reveals the weather we are experiencing. Presidential campaigns often are political barometers. They can, however, change the social climate. The Democratic presidential aspirants, plucking at the electorate’s sleeve, are perhaps inadvertently getting the country interested in moderation.

Many of them spent 2019 detailing how they would wield government as a scalpel to surgically remove America’s blemishes. Inconveniently, but without inhibiting these aspirants, the year ended with a cascade of discouraging headlines about the scalpel: “Overwhelmed by opium: The U.S. war on drugs in Afghanistan has imploded at nearly every turn” (The Post), “Withering Criticism of F.B.I. as Watchdog Presents Russia Inquiry Findings” (the New York Times), “Internal FAA Review Saw High Risk of 737 MAX Crashes” (the Wall Street Journal). Etc.

As 2020 begins, voters who say that their overriding objective is to remove the current president should wonder: Is it wise to hitch their wagons to any candidate whose agenda radiates, and requires, extravagant confidence in government’s ability to radically rearrange the United States’ most complex processes, from the allocation of economic resources to the provision of health care?

There is discouraging news for President Trump, which means encouraging news for the republic: Sen. Bernie Sanders (I-Vt.) is showing remarkable resilience, coronary and political, after his October “myocardial infarction.” This is encouraging not because the nation should swap one form of economic illiteracy for another, replacing a florid, angry protectionist with a florid, angry socialist. Rather, it is encouraging because Sanders and Sen. Elizabeth Warren (D-Mass.) continue to divide, more or less evenly, the “let’s give government lots more to do” constituency. The longer this balance persists, the more time that normal — meaning not very agitated or attentive — voters have to rally ’round candidates who do not make prudent people wince.

Now-former South Bend, Ind., mayor Pete Buttigieg, whose rise has been this political season’s most interesting development, might have the political magic of allowing different voters to see different things in him. This recalls Hamlet having fun with Polonius:

Hamlet: Do you see yonder cloud that’s almost in shape of a camel?

Polonius: . . . ’tis like a camel, indeed.

Hamlet: Methinks it is like a weasel.

Polonius: It is backed like a weasel.

Hamlet: Or like a whale?

Polonius: Very like a whale.

The Democrats’ nominee might be apparent late in the evening of March 3, “Super Tuesday,” when 14 states allocate about 34 percent of the delegates to the nominating convention. By that evening, Mike Bloomberg will have learned, to his delight or sorrow, whether political spending is as potent as campaign reform scolds believe, or whether there is a steeply declining utility of the last millions of political spending.

If the nominee will be known that evening, we are already in at least the seventh inning of the nomination game. But the top of the first inning can matter a lot.

In early December 2011, polls indicated that former Pennsylvania senator Rick Santorum, then seeking the Republican presidential nomination, was at 3 percent in polls anticipating the Jan. 3 Iowa caucuses. Granted, the vagaries of Iowa’s caucuses — typically long evening gatherings during a Midwestern winter — make predictive polling difficult. Still, Santorum’s support was negligible four weeks before he won with 24.6 percent. His wafer-thin victory was not confirmed until Jan. 20. If he, rather than Mitt Romney, had been announced the winner on caucus night, the subsequent competition might have developed differently.

So, it is still possible that someone — Colorado Sen. Michael F. Bennet, Minnesota Sen. Amy Klobuchar, New Jersey Sen. Cory Booker, former Maryland representative John Delaney, someone — might yet surge. Remember, as a song in “The Music Man,” which is set in Iowa, says, Iowans can be “contrary.” They might want to play “cheat the prophet,” as G.K. Chesterton described this game in “The Napoleon of Notting Hill”: People listen respectfully as experts explain the inevitable, then mischievously cause something else to happen.

Fortunately for those who seek entertaining surprises from politics, political science is a science of single instances, which means it is not much of a science. And fortunately for those who think today’s political fare is too highly seasoned, perhaps there is a market for something milder.

In Peter De Vries’s novel “I Hear America Swinging,” about the sexual revolution arriving in Iowa, the protagonist is invited to an orgy where he is nicknamed “Vanilla” because of his adherence to the sexually mundane. Perhaps in this year’s politics, as in ice cream year after year, the most popular flavor will be vanilla.

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