Humayun Khan was an American Muslim Army soldier who died serving the U.S. after 9/11. His father, Khizr Khan, spoke at the Democratic National Convention and offered a strong rebuke of Donald Trump, saying, "Have you even read the United States Constitution?" (Video: Victoria Walker/The Washington Post;Photo: Michael Robinson Chavez/The Washington Post)

Republican politicians face a choice. They can accept Hillary Clinton’s invitation to abandon Donald Trump and prevent a redefinition of their party as a haven for bigotry. Or they can prop Trump up, try to maximize his vote — and thereby tarnish themselves for a generation.

If there were any doubts about Trump’s disqualifying lack of simple decency and empathy, he resolved them on ABC’s “This Week” over the weekend with a characteristically cruel and self-centered attack on Khizr and Ghazala Khan, an American Muslim couple whose son, Army Capt. Humayun Khan, was killed in the line of duty in Iraq.

With his wife by his side, Khizr Khan delivered what was the most devastating attack on Trump during the Democratic National Convention. Khan directly challenged Trump’s strongman ignorance: “Let me ask you, have you even read the United States Constitution? I will gladly lend you my copy.” And he said this of Trump: “You have sacrificed nothing and no one.”

Most politicians — most human beings — would have humbly declared that no sacrifice is comparable to losing a son or daughter in service to the nation. Instead, Trump said he had made many sacrifices because (I’m not making this up) he “created thousands and thousands of jobs.” He said of Khizr Khan’s speech: “Who wrote that? Did Hillary’s script writers write it?”

And then he broke new ground, even for him, in heartlessness. “His wife,” Trump said, “if you look at his wife, she was standing there. She had nothing to say. She probably, maybe she wasn’t allowed to have something to say.”

Every Republican politician and commentator who continues to say that Trump is a superior or even morally equivalent choice to Hillary Clinton will now own their temporary leader’s brutality for the rest of their political careers.

Many humane Republicans know this. Ohio Gov. John Kasich spoke for them when he tweeted that “there’s only one way to talk about Gold Star parents: with honor and respect.”

This is a moment of truth for GOP leaders who passively accepted and sometimes encouraged an extremism that trafficked in religious and racial prejudice and painted President Obama as an illegitimate, power-hungry leader.

The party’s traditional chieftains assumed they could use these themes to rally an angry, aging base of white voters while keeping the forces of right-wing radicalism under control. They did not anticipate Trump. He spent years courting the far right with his charges that Obama was born abroad and set himself up in contrast to an establishment that cynically exploited its feelings.

Now, in Ronald Reagan’s revered phrase, comes a time for choosing. House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell look feeble and vacillating when they try to distance themselves from Trump outrages while maintaining their support for his election. They embody a calculating timidity as they worry about Trump’s impact on their party while also fearing for the electoral chances of their candidates if they push away Trump’s constituency.

Khizr Khan called their bluff in an interview Friday with MSNBC’s Lawrence O’Donnell: “This is a moral imperative for both leaders, to say to him, ‘Enough.’ ” Had Trump been watching, he would have known that Ghazala Khan also spoke out. Her reticence at the convention, she explained, arose from grief over her lost son that made it difficult even to talk about him.

She powerfully made her point again in an essay published online Sunday morning by The Post. As the outcry against Trump continued to build, Ryan issued a statement late Sunday afternoon declaring: “Many Muslim Americans have served valiantly in our military, and made the ultimate sacrifice,” adding that “Captain Khan was one such brave example.” But he continued practicing the politics of evasion: Ryan could not bring himself to mention Trump by name, let alone condemn him. McConnell issued a similar statement that also failed to name Trump.

Both Republicans and Democrats are publicly responding to the speech that Khizr Khan gave at the Democratic convention. Khan's son was killed while serving in Iraq. Here's what politicians from both parties said. (Monica Akhtar/The Washington Post)

Up to now, it has fallen largely to conservative intellectuals and former Republican officials to express their horror over Trump’s amoral approach to politics; his incoherent, dictator-friendly foreign policy; and his racist, exclusionary definition of what it means to be an American.

A Democratic convention awash in American flags celebrated American diversity in the name of a very old ideal most conservatives share: that we are a country bound by ideas, not a nation defined by blood, soil or a single religious tradition.

By contrasting Reagan’s “Morning in America” with Trump’s “Midnight in America,” Clinton invited such conservatives to tolerate a term of her leadership in order to avoid the damage a self-involved practitioner of a nasty brand of flimflam could do to their cause and their country.

Clinton Republicans and ex-Republicans could thus be this generation’s Reagan Democrats. In repudiating Trump for Clinton, they will not be abandoning their ideology. They will be making a moral statement that their movement will not tolerate an opportunist so corrupt and so vile that when given a choice, he pandered to religious intolerance rather than honoring the sacrifice of a brave young American.

Read more from E.J. Dionne’s archive, follow him on Twitter or subscribe to his updates on Facebook.