Steve Bannon. (Evan Vucci/Associated Press)

ALONG-STANDING practice of the modern White House is a division of labor between a chief of staff, who’s in charge of making the administrative trains run on time, and a political adviser, who counsels the president on the electoral consequences of his policies. In that sense, there’s nothing particularly new about President-elect Donald Trump’s decision to set up two lines of authority beneath him, one headed by Republican National Committee Chairman Reince Priebus as chief of staff, the other by erstwhile campaign chief Stephen K. Bannon as “chief strategist and senior counselor.”

What is extraordinary, and not in a good way, is the nature of the latter choice. Mr. Bannon is a leading figure on the euphemistically titled alt-right, a previously obscure element on the far fringe of conservatism — until Mr. Trump’s campaign energized and mainstreamed it. On its flagship website, Breitbart News, which Mr. Bannon dominates, the alt-right portrays itself as a middle-class uprising against a corrupt global elite. The movement can be more accurately described as deeply reactionary, rooted in a kind of white chauvinism, with disturbing overtones of anti-Semitism, visible in such Breitbart headlines as “Bill Kristol: Republican Spoiler, Renegade Jew,” or articles such as a recent attack on Post columnist Anne Applebaum as a “Polish, Jewish, American elitist.”

Mr. Bannon’s appointment sends a highly negative signal to all those Americans who did not support Mr. Trump for president but have been willing to give him the benefit of the doubt in deference to the legitimacy of his election. That is to say, his response to the impulse toward unity of the disappointed half of the United States has been to rub their noses in defeat by elevating a figure they understandably consider threatening.

Mr. Bannon may find himself unwelcome even in what remains of Republican establishment circles, given his extreme ideology and past attempts to foment GOP rebellions in the House of Representatives against Speaker Paul D. Ryan (Wis.) and his predecessor, John A. Boehner (Ohio). Republican leaders have welcomed the appointment of Mr. Priebus on the assumption that he can steer Mr. Trump toward mainstream policies and help restrain Mr. Bannon. Perhaps he will. On the other hand, neither Mr. Priebus nor most leading Republicans have proved able to check Mr. Trump’s worst impulses, to the extent they have been interested in doing so; over the past year, Mr. Priebus seemed more interested in playing an enabling, apologetic role for Mr. Trump. Greater strength than that will be needed to resist the forces of intolerance whose representatives are now moving into the White House.