The first error is to claim that Trump is the culmination and embodiment of Republicanism and conservatism. Republicans, in this view, have always been Trumpian but lacked Trump’s bluntness. Defeating Trump, the argument goes, would therefore crush the party’s true ideals: racism, social Darwinism, anti-intellectualism and corporate toadyism.

As a matter of substance, this claim is absurd. There may be some continuity among Trump, Mitt Romney, John McCain, George W. Bush and George H.W. Bush on environmental deregulation and opposition to socialized medicine. And elements of Trumpism can be discerned in Richard Nixon’s “Southern strategy” (an approach that we in the 2000 Bush campaign did our best to render irrelevant). But Trump’s direct appeal to racial resentment, his embrace of white grievance, his demonization of migrants, his abandonment of democratic ideals, his misogyny and xenophobia, his cruelty and menace require a separate category.

Trump’s break with past Republican leaders is not merely rhetorical. His divisiveness is foundational. He has ceased to pursue the common good as an organizing goal. He governs for the sake of himself and his Fox News-watching followers. His opponents are not merely mistaken; they are regarded as enemies who hate the country.

Trying to make Trump into a symbol of Republicanism is not only substantively absurd; it would also be strategically disastrous. There is no way to defeat Trumpism without isolating Trump. To frustrate his attempted redefinition of the right, and to redraw some important ethical and political lines, Trump must be seen as an aberration, as sui generis. Those who embrace his cause are not being good Republicans but bad ones. They are engaged in something novel and nasty.

A second Democratic error is like unto it. Some progressives seem to view Trump’s arrogant overreach as a golden opportunity for their own. They see the president’s weakness as a chance to run the ideological table and get the kind of “structural” change that is really a form of democratic socialism. Their argument here is less with Trump than with Barack Obama and Bill Clinton. Surveying the charred ruins of our public life, these leftists propose to fight fire with petrol.

Whatever ideologues may claim, politics is not about making the other side disappear. It is about building coalitions and making incremental progress. And so any party that seeks ultimate ideological victory is not merely encouraging division; it will find itself unable to succeed in actual politics, even when it wins elections.

Eventually one of the two parties will need to view the political weakness of the other side as a chance to expand its own coalition toward the center and put a halt to ideological escalation. In 2020, only one has the potential for such wisdom and flexibility (the other having descended into a personality cult). U.S. politics desperately needs a Democratic nominee who will (1) attack Trump as an aberration and (2) set out a humane agenda that will make achievable gains against durable problems. Our politics certainly doesn’t need another attempt to destroy the establishment by maximizing social and political chaos.

This conflict of visions within the Democratic Party may be setting up as a death match between Sen. Bernie Sanders (I-Vt.) and former vice president Joe Biden (or his ideological equivalent). Candidates on the center left have been making the case that progressive goals require realism about the means to achieve them. Health-care reform has been the test case. Biden and his kind have been arguing that Americans will not accept structural change from a radical and divisive figure. They are exactly right.

As a practical matter, Democrats will need early clarity on who will represent this viewpoint against Sanders or Massachusetts Sen. Elizabeth Warren (or against Sanders and Warren). Strong cases can be made for Biden (the decent and reassuring old hand), Minnesota Sen. Amy Klobuchar (the Midwestern pragmatist) or former South Bend, Ind., mayor Pete Buttigieg (the wunderkind). But after Iowa and New Hampshire, two from this group will need to display democratic heroism by endorsing the leading third. Otherwise we may be left with the same dynamic that bedeviled Republicans in 2016: scary intensity vs. fractured blandness.

This much is clear: A presidential contest between Trump and Sanders would not only be ideologically vivid. It would be the sign of a declining nation that can no longer produce leaders equal to its challenges.