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NINE DAYS from now, Americans who have not cast early ballots will go to the polls, finalizing what may be the most consequential midterm elections in modern U.S. history.

The high stakes in 2018 reflect the consequences of what happened in 2016: the election of Donald Trump as president. Contrary to many wishful forecasts, assuming power did not render Mr. Trump more responsible or temperate, more respectful of long-standing norms or even of the ordinary human feelings of those with whom he interacts at home and abroad. He has used the bully pulpit of the presidency to normalize vulgarity and venom and to divide an already divided nation along lines of race, region, gender and education. He has done repeated violence to the truth, about matters trivial (the size of the crowd at his inauguration) and tragic (the presence of “very fine people, on both sides,” during last year’s violence-plagued right-wing demonstrations in Charlottesville). As we have repeatedly emphasized, this is not what a normal president would do. It is not what any decent person would do.

Alas, Mr. Trump has maximized the political advantage to be gained from this debasement of politics. His popularity is not high, but, among the 40-odd percent of the country that kindles to his message, it is strong. The president is now campaigning around the nation asking more voters to join the “base” on whose behalf he preferentially governs. In the face of Democratic calls to treat what is actually a race for House, Senate, governor and myriad other offices as a referendum on Mr. Trump, the president responds, in effect, “Bring it on.” Rather than modulate his divisive rhetoric against immigration, the media and other bogeymen, he has doubled down.

As a result, the country is passing through a political storm even though economic conditions are not only benign but prosperous. In an ordinary year, that continuing expansion might be at the center of political debate: To what degree does Mr. Trump deserve credit or — given the massive run-up of federal deficits — blame? In 2018, however, pocketbook issues must not override legitimate concerns about the health of our political institutions.

Inescapably, the issue that predominates is Mr. Trump’s noxious, divisive and dishonest style of government. Fearmongering over a distant band of ragtag Honduran migrants. A fairy tale about a middle-class tax cut. Relentless vilification of essential institutions of democracy, including an independent Federal Reserve, the media, organs of law enforcement and the opposition party. Resistance to legitimate judicial and congressional oversight. Restricting the franchise under the pretext of rampant voter fraud: He has put all of these on the ballot. Sadly, most members of his party have opportunistically gone along.

Inevitably, the politicians who oppose Trumpism are themselves, in various ways and to varying degrees, imperfect. Nevertheless, we believe voters should back any candidate who will stand up to Mr. Trump’s brand of reactionary populism. After Nov. 6, we will have a better idea whether 2016 was the beginning of an extended dark period in U.S. politics or an aberration that shocked the nation’s democrats, of whatever party affiliation, into effective action. Think about that, and vote accordingly.