In the age of Donald Trump, it is a British comic with a mild regional accent and an acerbic wit who is the most popular satirist on American television.

John Oliver, 39, returns to screens this Sunday with a fourth season of HBO's award-winning "Last Week Tonight" at a time when the Trump presidency stands accused of testing the limits of freedom of expression and a free press.

Oliver, probably more than anyone else, has stepped into the shoes of Jon Stewart, the American satirist who transformed US political comedy before retiring from "The Daily Show" in 2015 just as the Trump train got started.

It was "The Daily Show" that gave Oliver his big break, plucking him from obscurity in England to work on the show from 2006 to 2014, when he appeared in a somber suit with rumpled British-rocker hair for reporter-style segments.

He even filled in for Stewart in 2013 while the American worked on a film project. HBO then offered him his own show.

"Last Week Tonight" launched in 2014. Sitting behind a desk, again in a somber suit, he has delivered verbal punches on weighty topics such as automobile lenders, charter schools and food waste.

It turned him into a voice that counts. It stood him in good stead for the political arrival of Trump, with one particular segment about his campaign viewed 31 million times on YouTube.

"It was quite hard last year," Oliver told AFP in an interview in New York.

"Normally you take something serious and then you make it silly. But if you have something already stupid, how do you show people that that's actually more important than it sounds? That was the problem," he said.

Now that Trump is in power, Oliver expects it to be difficult in a different way. He calls some of the language coming out of the White House "objectively dangerous" -- such as the president calling the press the opposition.

- 'Sucked down' -

But he is also wary of devoting too much time to the Republican billionaire-turned-world leader.

Last year, Trump and the election were the bulk of just eight out of 30 shows, and this season, Oliver says he wants "to make sure that we don't get sucked down the easy road of making everything about Trump."

The program has been working on stories that have nothing to do with the administration, even if tangentially almost everything is "because the solution to any potential problem has to run through the White House," he said.

As with last year, when he tackled Brexit and the crisis in Brazil, Oliver wants again to give viewers a sense of what is happening abroad, the French presidential election and far-right candidate Marine Le Pen included.

"I don't think people understand what that name means here. So yes, I'm fascinated with the French election. We will absolutely be looking at that," he said.

- 'That line' -

While Oliver doesn't exactly hide his left-leaning tendencies, he likes to believe that the rigor of his approach plays beyond his naturally Democratic-centric audience.

He points out, for example, that a segment on civil forfeitures got a lot of positive feedback on conservative websites.

"I would hope that even people that are diametrically opposed to how I might be politically might still find interesting things in the show," he says.

"They wouldn't be able to say 'the facts are wrong' but they can absolutely disagree with my conclusions to those."

Though relatively few Britons break through into American television, Oliver is particularly unusual in having been relatively unsuccessful in England. His home and his career are here now, he says.

"In general, comedians are outsiders. It's an outsider eye. And it probably helps being from a different country."

If his work is often compared to journalism, it is a comparison that Oliver shies away from.

"It's pretty clearly not journalism," he says, stressing it is instead dependent on the work of investigative reporters.

"I'm not qualified," he says. "We need people to have done that reporting and that reporting to appear on television so we can do clips about it."

He is also aware that the freedom of expression enshrined in the US Constitution and HBO's ad-free model gives him unusual latitude in being able to pick his targets, large corporations included.

"That's why I feel even more of a duty to swing as hard as I can. I get all the toys. So I don't feel you get to feel lazy about that."

"I'll keep trying to find where that line is," he said.