In his 33 years as a late-night entertainer, David Letterman was both a host and an antagonist to politicians of every stripe. He took a centrist approach to NBC's Late Night (which he starred in from 1982 to 1993) and at CBS' Late Show (where he was the master of ceremonies from 1993 until his retirement in 2015), holding wide-ranging conversations with President Bill Clinton or Senator John McCain on a given evening, then bringing the satirical hammer down on them the following night.

Hillary Clinton, now the Democratic presidential nominee, was a frequent guest as well as a comedic target. During her failed presidential bid in 2008, for example, he joked that her campaign was "$20 million in debt - now when she gets that three am call, it's from a collection agency."

Donald Trump on The Late Show with David Letterman.

Letterman, 69, had an especially feisty relationship with Donald Trump, who had been a frequent guest and sparring partner since the 1980s. As an interview subject since becoming the Republican presidential nominee, Trump has since bedeviled hosts like Jimmy Fallon (who was criticised for going too easy on him) and Stephen Colbert (who acknowledges he should have been a tougher interrogator). But Letterman seemed to land some lasting hits on Trump: a 2012 Late Show segment in which he mocked Trump for outsourcing his Macy's clothing line to Asia resurfaced this year in a campaign ad for Clinton.

Now Letterman says Trump is no longer a laughing matter. Discuss his involvement in the National Geographic Channel series Years of Living Dangerously, which examines environmental problems around the world, Letterman also spoke about his evolving thoughts on the Republican nominee and why he considers him "a damaged human being" and "a person to be shunned." These are edited excerpts from that conversation.