Danielle Allen is a political theorist at Harvard University and a contributing columnist for The Post.

Like any number of us raised in the late 20th century, I have spent my life perplexed about exactly how Hitler could have come to power in Germany. Watching Donald Trump’s rise, I now understand. Leave aside whether a direct comparison of Trump to Hitler is accurate. That is not my point. My point rather is about how a demagogic opportunist can exploit a divided country.

To understand the rise of Hitler and the spread of Nazism, I have generally relied on the German-Jewish émigré philosopher Hannah Arendt and her arguments about the banality of evil. Somehow people can understand themselves as “just doing their job,” yet act as cogs in the wheel of a murderous machine. Arendt also offered a second answer in a small but powerful book called “Men in Dark Times.” In this book, she described all those who thought that Hitler’s rise was a terrible thing but chose “internal exile,” or staying invisible and out of the way as their strategy for coping with the situation. They knew evil was evil, but they too facilitated it, by departing from the battlefield out of a sense of hopelessness.

[George Will: Trump relishes wrecking the GOP]

One can see both of these phenomena unfolding now. The first shows itself, for instance, when journalists cover every crude and cruel thing that comes out of Trump’s mouth and thereby help acculturate all of us to what we are hearing. Are they not just doing their jobs, they will ask, in covering the Republican front-runner? Have we not already been acculturated by 30 years of popular culture to offensive and inciting comments? Yes, both of these things are true. But that doesn’t mean journalists ought to be Trump’s megaphone. Perhaps we should just shut the lights out on offensiveness; turn off the mic when someone tries to shout down others; reestablish standards for what counts as a worthwhile contribution to the public debate. That will seem counter to journalistic norms, yes, but why not let Trump pay for his own ads when he wants to broadcast foul and incendiary ideas? He’ll still have plenty of access to freedom of expression. It is time to draw a bright line.

Donald Trump won the Feb. 20 South Carolina GOP primary. Here's how. (Peter Stevenson/The Washington Post)

One spots the second experience in any number of water-cooler conversations or dinner-party dialogues. “Yes, yes, it is terrible. Can you believe it? Have you seen anything like it? Has America come to this?” “Agreed, agreed.” But when someone asks what is to be done, silence falls. Very many of us, too many of us, are starting to contemplate accepting internal exile. Or we joke about moving to Canada more seriously than usually.

But over the course of the past few months, I’ve learned something else that goes beyond Arendt’s ideas about the banality of evil and feelings of impotence in the face of danger.

[Five myths Donald Trump tells about Donald Trump]

Trump is rising by taking advantage of a divided country. The truth is that the vast majority of voting Americans think that Trump is unacceptable as a presidential candidate, but we are split by strong partisan ideologies and cannot coordinate a solution to stop him. Similarly, a significant part of voting Republicans think that Trump is unacceptable, but they too, thus far, have been unable to coordinate a solution. Trump is exploiting the fact that we cannot unite across our ideological divides.

The only way to stop him, then, is to achieve just that kind of coordination across party lines and across divisions within parties. We have reached that moment of truth.

Republicans, you cannot count on the Democrats to stop Trump. I believe that Hillary Clinton will win the Democratic nomination, and I intend to vote for her, but it is also the case that she is a candidate with significant weaknesses, as your party knows quite well. The result of a head-to-head contest between Clinton and Trump would be unpredictable. Trump has to be blocked in your primary.

Jeb Bush has done the right thing by dropping out, just as he did the right thing by being the first, alongside Rand Paul, to challenge Trump. The time has come, John Kasich and Ben Carson, to leave the race as well. You both express a powerful commitment to the good of your country and to its founding ideals. If you care about the future of this republic, it is time to endorse Marco Rubio. Kasich, there’s a little wind in your sails, but it’s not enough. Your country is calling you. Do the right thing.

Ted Cruz is, I believe, pulling votes away from Trump, and for that reason is useful in the race. But, Mr. Cruz, you are drawing too close to Trump’s politics. You too should change course.

Democrats, your leading candidate is too weak to count on as a firewall. She might be able to pull off a general election victory against Trump, but then again she might not. Too much is uncertain this year. You, too, need to help the Republicans beat Trump; this is no moment for standing by passively. If your deadline for changing your party affiliation has not yet come, re-register and vote for Rubio, even if, like me, you cannot stomach his opposition to marriage equality. I too would prefer Kasich as the Republican nominee, but pursuing that goal will only make it more likely that Trump takes the nomination. The republic cannot afford that.

[Catherine Rampell: Donald Trump thinks you’re stupid. Yes, you.]

Finally, to all of you Republicans who have already dropped out, one more, great act of public service awaits you. As candidates, you pledged to support whomever the Republican party nominated. It’s time to revoke your pledge. Be bold, stand up and shout that you will not support Trump if he is your party’s nominee. Do it together. Hold one big mother of a news conference. Endorse Rubio, together. It is time to draw a bright line, and you are the ones on whom this burden falls. No one else can do it.

Marco Rubio, this is also your moment to draw a bright line. You too ought to rescind your pledge to support the party’s nominee if it is Trump.

Donald Trump has no respect for the basic rights that are the foundation of constitutional democracy, nor for the requirements of decency necessary to sustain democratic citizenship. Nor can any democracy survive without an expectation that the people require reasonable arguments that bring the truth to light, and Trump has nothing but contempt for our intelligence.

We, the people, need to find somewhere, buried in the recesses of our fading memories, the capacity to make common cause against this formidable threat to our equally shared liberties. The time is now.