Charles Koch, the billionaire industrialist and conservative megadonor, made a rare public appearance Thursday evening and called for libertarians to embrace their principles, in comments that seemed obliquely directed at a Republican Party taken over by President Trump.

Mr. Koch was at one time among the most powerful forces in Republican politics. In the 2016 election cycle alone, he and his allies spent $750 million to promote the party’s candidates and causes. But his political power has waned significantly since Mr. Trump’s election that year, and he is now seldom seen in Washington. And neither do Republicans worry much about his plans in a party that is much more in Mr. Trump’s image than in Mr. Koch’s.

But Mr. Koch, who will turn 90 this November, showed up in Washington to accept an award from the Cato Institute. Almost 50 years ago, Mr. Koch helped found Cato, one of the nation’s prominent libertarian think tanks. By 2012, Mr. Koch and his brother David had given about $30 million to the institute, but the relationship soured and the Kochs ended up suing the nonprofit before settling that June.

Accepting a prize named after Milton Friedman, the free-market economist, on Thursday, Mr. Koch made his first public remarks since Mr. Trump was inaugurated in January and enacted a number of policies that are anathema to Mr. Friedman’s and Mr. Koch’s politics, most notably the sweeping tariffs.

Mr. Koch dispensed with the cheery rhetoric of most conservatives these days. Speaking about the subsidies and protectionism of the past, he said that “you can see why we’re in the mess we’re in today.” The billionaire often speaks about his core “principles” in business and philanthropy.

“With so much change, chaos and conflict, too many people and organizations are abandoning these principles,” Mr. Koch later said, not uttering Mr. Trump’s name. He added, “But we know from history, this just makes the problems worse. And people have forgotten that when principles are lost, so are freedoms.”

Mr. Koch’s political group announced a $20 million campaign this year to try to encourage the renewal of Mr. Trump’s 2017 tax cuts. But Mr. Koch has made no secret of his profound disagreements with the president. In 2024, the Koch network took a more muscular approach, but to no avail, endorsing Nikki Haley in her bid for the Republican nomination. “CHARLES KOCH AND HIS GROUP GOT PLAYED FOR SUCKERS RIGHT FROM THE BEGINNING!” Mr. Trump posted as Ms. Haley’s campaign sputtered.

Mr. Koch still has financial capital, even if not always political: A super PAC tied to Mr. Koch spent almost $180 million on the 2024 cycle, a sum that does not include spending by affiliated nonprofit groups. Mr. Koch has a net worth of $66 billion, according to the Bloomberg Billionaires Index, making him the 22nd wealthiest person in the world, just behind the family of Julia Flescher Koch, the widow of David Koch, who died in 2019. Mr. Koch’s firm, Koch Inc., is the second-largest privately owned company in the country (Cargill is No. 1).

Mr. Koch’s speech in a Washington ballroom made for his most prominent engagement since at least the pandemic. He has done a few podcasts and interviews as late as early 2022 to promote his 2020 book, but he has largely ceded the spotlight to his son, Chase Koch.

Approaching his tenth decade, Mr. Koch still speaks volubly and moves well. When his speech was interrupted by protesters, Mr. Koch was not fazed.

“When you have those kind of enemies, you know you’re doing the right thing,” he said.