Mitt Romney speaks about Donald Trump in Salt Lake City on Thursday. (Jim Urquhart/Reuters)

TRUE, MITT Romney is an imperfect messenger for the denunciation of Donald Trump that he delivered Thursday. Mr. Romney cheerfully accepted a Trump endorsement in 2012. As the former standard-bearer of his party, he is a quintessential establishment figure, whose criticism may be taken as affirmation by some Trump supporters. And, yes, he is coming late to the argument.

Nevertheless, he was right to deliver his truth-telling message, and we hope others will join in.

Mr. Trump has ridden atop the polls for months, seemingly every day finding some new person or group of people to denigrate — women, people with disabilities, Muslims, immigrants, reporters, prisoners of war. He has attacked essential freedoms, threatened trouble for those who disagree with him and made promises that seem to ignore constitutional checks and balances. Republican leaders have mostly kept quiet, daring to condemn a couple of Mr. Trump’s most outrageous statements and then retreating.

By contrast, Mr. Romney offered a comprehensive, substantive and compelling case against the billionaire bloviator. Importantly, his critique went well beyond simply arguing that Mr. Trump is not a true conservative. Mr. Romney started by pointing out that Mr. Trump is not the “genius” he makes himself out to be — in running a business or running the country. By instigating trade wars with China, Mexico and no doubt others, Mr. Trump would sink the country into a “prolonged recession,” Mr. Romney argued. He would destroy the country’s export sector, explode the deficit and harm national security by, among other things, alienating the moderate Muslims the United States needs as allies.

But even his terrible policies, such as anyone can understand them, are not the primary reason Mr. Trump should be opposed. What is most dangerous is Mr. Trump’s bigoted, anti-democratic tendencies. Mr. Trump “creates scapegoats of Muslims and Mexican immigrants, he calls for the use of torture and for killing the innocent children and family members of terrorists. He cheers assaults on protesters. He applauds the prospect of twisting the Constitution to limit First Amendment freedom of the press. This is the very brand of anger that has led other nations into the abyss.”

The threat Mr. Trump represents transcends party or ideology, and it cannot be tolerated.

“Watch how he responds to my speech today,” Mr. Romney advised. “Will he talk about our policy differences, or will he attack me with every imaginable low-road insult? This may tell you what you need to know about his temperament, his stability and his suitability to be president.”

Not surprisingly, Mr. Trump could not help himself. Recounting how Mr. Romney wanted his endorsement in 2012, the bully candidate said, “I could’ve said, ‘Mitt drop to your knees,’ and he would’ve dropped to his knees.”

All you need to know, indeed.