Former NBC TV host Billy Bush has confirmed that he heard Donald Trump boasting about grabbing women by the genitals in a 2005 recording.
In the New York Times, he said the US president was "indulging in revisionist history", amid reports Mr Trump now questioned whether the tape was real.
Mr Trump originally apologised for the remarks, which came to light a month before he won last year's election.
Top Republicans condemned the tape, but most still backed Mr Trump's campaign.
In his opinion piece, the former co-anchor of NBC's Access Hollywood show said he and seven others who had witnessed the remarks were convinced they were "listening to a stand-up act".
"He said it. 'Grab 'em by the pussy,'" Bush wrote.
"Of course he said it. And we laughed along, without a single doubt that this was hypothetical hot air from America's highest-rated bloviator."
But Bush said that after reading accounts by 20 women who have come forward to accuse Mr Trump of unwanted sexual advances, he now knew better.
"President Trump is currently indulging in some revisionist history, reportedly telling allies... that the voice on the tape is not his. This has hit a raw nerve in me," Bush added.
He said the president was "wantonly poking the bear" as the US was trying to come to terms with years of sexual abuse and misconduct against leading figures in the media and entertainment industry.
Mr Trump has denied any allegations against him of sexual misconduct and threatened to sue his accusers.
But at the time the videotape was released he appeared to acknowledge that it was real, dismissing it as "locker room talk".
According to US media reports, Mr Trump has been privately telling allies that the voice on the recording is not his.
Bush, a nephew of former president George HW Bush, lost his job as co-host of NBC's Today show after the videotape was released.