A leading host on Fox News, a conservative network notorious for its loyalty to the White House, has lambasted Donald Trump for mounting the most direct attack on press freedom in American history.

Chris Wallace, widely admired for breaking ranks from Fox colleagues by putting tough questions to administration officials, delivered his most stinging critique yet of the US president at an event celebrating the first amendment.

“I believe that President Trump is engaged in the most direct sustained assault on freedom of the press in our history,” Wallace said to applause at the Newseum, a media museum in Washington, on Wednesday night.

“He has done everything he can to undercut the media, to try and delegitimise us, and I think his purpose is clear: to raise doubts when we report critically about him and his administration that we can be trusted. Back in 2017, he tweeted something that said far more about him than it did about us: ‘The fake news media is not my enemy. It is the enemy of the American people.’”

Wallace recalled that retired admiral Bill McRaven, a navy Seal for 37 years, had described Trump’s sentiment as maybe “the greatest threat to democracy in my lifetime” because, unlike even the Soviet Union or Islamic terrorism, it undermines the US constitution.

The veteran broadcaster added: “Let’s be honest, the president’s attacks have done some damage. A Freedom Forum Institute poll, associated here with the Newseum, this year found that 29% of Americans, almost a third of all of us, think the first amendment goes too far. And 77%, three quarters, say that fake news is a serious threat to our democracy.”

Chris Wallace was speaking at an event in Washington. Photograph: AWNewYork/REX/Shutterstock

Wallace is a rare dissenting voice at the Rupert Murdoch-owned Fox News, where opinion hosts such as Sean Hannity and Laura Ingraham are fiercely pro-Trump. Longtime anchor Shep Smith, who was also praised for his independence, stepped down in October and warned that “intimidation and vilification of the press is now a global phenomenon. We don’t have to look far for evidence of that.”

Wallace, son of distinguished journalist Mike Wallace, conducts some of the sharpest grillings of any of America’s long running Sunday politics shows. When he recently took House minority whip Steve Scalise to task, Trump responded with a tweet that called Wallace “nasty” and “obnoxious”.

But at Wednesday’s event, a farewell to the Newseum which is closing down after nearly 12 years at its current location, Wallace also warned the media against overreach. “I think many of our colleagues see the president’s attacks, his constant bashing of the media as a rationale, as an excuse to cross the line themselves, to push back, and that is a big mistake,” he said.

“I see it all the time on the front page of major newspapers and the lead of the evening news: fact mixed with opinion, buzzwords like ‘bombshell’ and ‘scandal’. The animus of the reporter and the editor as plain to see as the headline.”