Republican presidential candidate Donald Trump speaks during a campaign event at the Myrtle Beach Convention Center on Tuesday. (Willis Glassgow/Associated Press)

JUST WHEN you think Donald Trump’s campaign could not possibly become more loathsome, the billionaire bully proves you wrong again. Mr. Trump’s latest ugliness was to mock the disability of a newspaper reporter who had displeased him. Then — and this is the least surprising part off the story — he lied about it.

Lies are the foundation of the Trump campaign, and of this latest episode. He took off in the polls when he characterized Mexican immigrants as rapists, and he has been spreading untruths ever since. His lies are not random: They are calculated to inflame and exploit voter fears and prejudices. Thus, he retweets falsehoods about how many white people are murdered by black people; he trumpets falsehoods about how many Syrian refugees President Obama wants to admit into the United States. Recently he has been recycling an Internet myth about “thousands” of American Muslims celebrating the attack on the World Trade Center towers in 2001.

New York Times reporter Serge F. Kovaleski, who was covering the attacks and their aftermath for The Post at the time, offended Mr. Trump by disputing this latest lie, noting, as have many others, that there were no mass celebrations. This was important because Mr. Trump was citing an article written by Mr. Kovaleski as his purported source. Since Mr. Kovaleski’s credibility is rock-solid — he is one of the most highly respected journalists of his generation — Mr. Trump could only respond with his usual juvenile playground taunts. Mr. Kovaleski has a condition called arthrogryposis, which limits and affects his arm movements, so Mr. Trump mocked him by jerking his arms about as he said, “Now, the poor guy — you’ve got to see this guy, ‘Ah, I don’t know what I said! I don’t remember!’ ”

When rebuked, Mr. Trump doubled down on the lies, saying he hadn’t been mocking Mr. Kovaleski and doesn’t even know what he looks like. This was obviously false (“you’ve got to see this guy”) but that doesn’t matter to the candidate. In the course of his sad campaign, he has denigrated women, people who are overweight, immigrants, Muslims and most of his rivals, in the most personal terms. Does he prepare his insults, or do they just spew forth as Mr. Trump faces the adoring crowds that he has believed, all his life, he deserves? There’s probably some of both.

Groups advocating for the rights of people with disabilities condemned Mr. Trump’s remarks. That’s fine, but not sufficient. As we have said before, it is time for Republican Party leaders to make clear that they do not approve of Mr. Trump’s politics of denigration. If they do not, their party will be seen as complicit in his hatefulness, and deservedly so.