DURHAM, N.H.

It’s one of the ironies of the Donald Trump era. Saddled with the most polarizing president in modern history, the Republican Party finds itself uncommonly united.

Yet as they prepare to face off against that same president, the Democrats are sharply divided — and that division will likely get worse before it gets better.

The GOP’s unity is a testament both to the plasticity of conservative principles and the mesmerizing hold Trump has on grass-roots Republicans. Today’s GOP is Trump’s party, its tenets pretty much whatever he declares them to be.

The Democrats’ problem is the near opposite: There are deep differences between the center-left and the farther left about the way forward. The basic question: Should Democrats stick with Barack Obama-style liberalism or embrace something more radical?

Senator Bernie Sanders of Vermont wants to make the party the delivery vehicle for his unwavering democratic socialist vision. If he prevails in the primaries, his platform would be well to the left of anything a Democratic nominee has run on before. Although she’s somewhat to Sanders’ starboard, Senator Elizabeth Warren is offering a more redistributive progressivism than anything since LBJ’s Great Society.

Center-left types worry a Sanders candidacy would subject the party to a Jeremy Corbyn-style defeat, making it nigh unto impossible for Democrats to retake the Senate and perhaps even to hold the House.

But at a Monday rally that drew some 7,500 fervent Sanders supporters to the University of New Hampshire’s Whittemore Center Arena, the message from Bernie and such prominent backers as Alexandria Ocasio-Cortez, Cornell West, and Cynthia Nixon was this: Don’t listen to electability concerns. As West put it, these democracy-testing times require a stalwart like Sanders, “not a milquetoast neoliberal.’’

Earlier in the day, at an Amy Klobuchar event in Exeter, some retirement-age voters were still trying to decide which Democrat has the best shot at beating Trump. No such campaign calculations fretted the foreheads at the Bernie-and-two-rock-bands rally, at least not among the college-age and later-20s types I talked with. They were confident Sanders could win. And give them this: National polling regularly shows the Vermont independent senator beating Trump.

At the Klobuchar event, I asked attendees if they would back Sanders should he win the nomination. To a person, the answer was an immediate anyone-but-Trump affirmative. At UNH, however, the Sanders crowd was more hesitant about supporting a non-Sanders nominee. To be sure, some were a quick yes. Probably, as long as Sanders was treated fairly along the way, was another answer I got more than once. But several others said they or their friends would probably not vote if Sanders weren’t the nominee.

Now, it’s incumbent on the candidates to keep the political debate civil enough that supporters of unsuccessful rivals aren’t irrevocably alienated. Although the belittling tone a recent Joe Biden ad took toward Pete Buttigieg’s mayoral record was unfortunate, so far they have.

But it’s not the candidates alone who bear a responsibility for post-primary unity. So, too, do Democratically inclined voters. Passionate supporters have an unfortunate tendency to see their own favorite as the very embodiment of truth and righteousness and to disdain his or her rivals as frauds.

The propensity for political purism increases the further one moves toward either end of the spectrum. On the right, conservatives regularly disparage Republican moderates or Trump dissenters as RINOs, while ardent leftists often see center-left candidate as shams or sell-outs. We’ve all heard the rhetoric: There’s really no difference between moderate Democrats and Republicans; they are all controlled by special-interest money or lobbyists, all part of the corporatist Washington consensus.

That’s not just intellectually lazy, it’s immensely and demonstrably wrong.

From climate change to health care to gun control to DACA to the Supreme Court to the integrity of democratic institutions and norms, there’s too much at stake this time around to indulge in political pique or sulkiness or indolence.

Yes, the candidates themselves must do their utmost to unify the party once the nominee emerges.

But everyday voters must respond in kind. These times require that everyone put disappointment aside and become instant electoral adults.

Scot Lehigh is a Globe columnist. He can be reached at scot.lehigh@globe.com. Follow him on Twitter @GlobeScotLehigh.