By Editorial Board – The New York Times.

The Republican Party’s trek into the darkness took a fateful step in Indiana on Tuesday.

The Hoosier State delivered an all-but-crowning victory to Donald Trump, who beat Ted Cruz soundly in the state, sweeping up at least 51 delegates. On the Democratic side, Bernie Sanders won an unexpected victory over Hillary Clinton, though it was not enough to halt her march to the nomination.

Shortly after the Republican race was called, Mr. Cruz announced that he was ending his campaign, leaving Gov. John Kasich as the sole rival to Mr. Trump in the G.O.P. contest.

That the Never-Trumpers had hoped to fall back on Mr. Cruz, perhaps the most reviled politician in his party, was a measure of their panic about the prospect now before them. With Mr. Trump’s success, “I’m watching a 160-year-old political party commit suicide,” said Henry Olsen, an elections analyst with the Ethics and Public Policy Center, a conservative think tank.

Republicans will all but certainly nominate Mr. Trump, who would be the most volatile and least prepared presidential candidate nominated by a major party in modern times. A man once ridiculed by many prominent Republicans will become the G.O.P. standard-bearer.

This is a moment of reckoning for the Republican Party. It’s incumbent on its leadership to account for the failures and betrayals that led to this, and find a better way to address them than the demagogy on offer.

Republicans haven’t yet begun to grapple with this. Instead they’re falling into line.

Republican leaders have for years failed to think about much of anything beyond winning the next election. Year after year, the party’s candidates promised help for middle-class people who lost their homes, jobs and savings to recession, who lost limbs and well-being to war, and then did next to nothing. That Mr. Trump was able to enthrall voters by promising simply to “Make America Great Again” — but offering only xenophobic, isolationist or fantastical ideas — is testimony to how thoroughly they reject the politicians who betrayed them.

Now, myopic as ever, Republican leaders are talking themselves into supporting Mr. Trump. At a party retreat in Florida last month, Mr. Trump’s adviser Paul Manafort, brought in to make the candidate seem safer to the old guard, assured them that Mr. Trump will better prepare himself for the presidency. “That was all most of these guys needed to hear,” said an operative in the room. “Maybe he’s trainable.” But within a day, Mr. Trump was back to making vile comments at his rallies. In his confused foreign policy address, he demonstrated nothing but a willful refusal to learn.

Some Republicans still seem to hope they can direct voters’ attention past the Trump candidacy. Last week in Washington, Paul Ryan, the speaker of the House, told a dismayed young Republican at Georgetown University to try not to worry so much about Mr. Trump. “I would just ask you to raise your gaze and look at the horizon that we’re trying to paint,” he said, promoting #ConfidentAmerica, his plan to create a plan. Its mission reads like this: “We do not like the direction the country is going, and we have an obligation to offer an alternative. That’s why House Republicans are developing a bold, pro-growth agenda to take to the country. By giving the people a clear choice in 2016, we can earn a mandate to do big things in 2017 and beyond.”

It is the Republicans who are making a clear choice in 2016, one that seemed unimaginable a year ago: To stamp what they still like to call the party of Lincoln with the brand of Donald Trump.