Former White House chief strategist Steve Bannon says the firing of FBI Director James Comey may have been the biggest mistake in "modern political history."

In a CBS interview on 60 Minutes that aired Sunday, Bannon confirmed he opposed President Donald Trump's decision to oust Comey, calling the FBI "an institution."

Bannon said institutions such as the U.S. Senate and House of Representatives can be changed "if the leadership is changed." But he also said the FBI is different.

"I don't believe that the institutional logic of the FBI, and particularly in regards to an investigation, could possibly be changed by changing the head of it," Bannon said.

He also said that if Comey hadn't been fired, "we would not have the Mueller investigation," referring to special counsel Robert Mueller.

Comey testified before Mueller and the Senate intelligence committee in June. Mueller is now reportedly examining a range of alleged incidents including obstruction of justice and money laundering, and numerous White House staff and presidential associates have hired lawyers.

Bannon offered no comment when asked whether the firing was supported by one of his political nemeses: the president's son-in-law, Jared Kushner.

Kushner is among the numerous rivals Bannon clashed with in the White House. 

In the 60 Minutes interview, Bannon sought to settle scores with a few of them. He suggested economic adviser and Democrat Gary Cohn should resign, rather than complaining publicly about the way Trump handled the racial incident in Charlottesville, Virginia, in August. 

Cohn, in an interview with the Financial Times, had sharply denounced Trump for saying that "many sides" were to blame for the violence in Charlottesville, Va., and criticized the administration's response to incident.

Bannon also accused the Republican Party leadership of trying to block Trump's agenda.

The congressional wing of the party is more supportive of trade deals like NAFTA, more favourable to immigration, and less supportive of funding a wall with Mexico, than Trump is.

"The Republican establishment is trying to nullify the 2016 election," Bannon told interviewer Charlie Rose.

"That's a brutal fact we have to face... I think [Senate Leader] Mitch McConnell, and to a degree, [House Leader] Paul Ryan, they do not want Donald Trump's populist, economic nationalist agenda to be implemented. It's very obvious."

Bannon suggested a current split over undocumented children could rip the party apart. He predicted a nationalist, populist movement will prevail in American politics — but it's not yet clear whether it will be of a left-wing or right-wing variety.

He said that depends on whether Republicans or Democrats take up the cause of trade skepticism.

Bannon, who has returned to his role as head of right-wing media organization Breitbart, insisted he will continue to support Trump's agenda against the pro-trade, pro-globalization Republican establishment.

With files from The Canadian Press