Steve Bannon, Donald Trump’s former chief strategist, has launched a war against the Republican establishment and party leaders.

Mr Bannon, who left the White House last month, is said to be planning to attack mainstream Republican senators seeking re-election in 2018.

After returning to his perch at Breitbart News, a far-right media outlet, Mr Bannon reportedly met with those seeking to challenge Republican incumbents in next year’s primaries, according to Politico.

The news organisation also reported that Mr Bannon is working with conservative mega-donor Robert Mercer, who is apparently ready to pour millions of dollars into campaigns assailing establishment Republicans.

Some likely targets of Mr Bannon’s attacks include Senator Jeff Flake of Arizona, an outspoken critic of the President, and Senator Dean Heller of Nevada, who had refused to endorse Mr Trump during the 2016 campaign.

In the Alabama Senate special election, Mr Bannon has backed Roy Moore – a former state Supreme Court Justice and evangelical voter favourite – rejecting Senator Luther Strange, who Mr Trump has endorsed and is being supported by allies of Senate Majority Leader Mitch McConnell.

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Senior Republicans have expressed concern that Mr Bannon’s attacks during primaries could drain precious resources needed to compete against Democrats in the general election. Currently, Mr McConnell has a narrow four-seat majority in the 100-member Senate.

While not directly discussing his efforts to dislodge incumbent Republican senators, Mr Bannon told CBS’s Charlie Rose on 60 Minutes that “the Republican establishment is trying to nullify the 2016 election.”

Steve Bannon says the firing of James Comey was the biggest mistake in modern political history

“That's a brutal fact we have to face,” he added, saying that Mr McConnell and to a degree, House Speaker Paul Ryan, “do not want Donald Trump's populist, economic nationalist agenda to be implemented.”

“It's very obvious,” he said.

When asked if Mr Trump agreed with that characterisation of Mr McConnell and Mr Ryan, White House Press Secretary Sarah Huckabee Sanders said during her briefing, “The President is committed to working with Congress to get some big things done.”

“We've got a very big agenda. The President wants to work with all members of Congress. Obviously that includes Republican leadership, as well as Democrats,” she said, referencing the President's deal last week with Senator Charles Schumer and Representative Nancy Pelosi, the highest-ranking Democrats in Congress, to provide aid to Hurricane Harvey victims, increase the debt limit and fund the government until 15 December.

During his interview with Mr Bannon, Mr Rose told the media executive: “You are attacking on many fronts people who you need to help you to get things done.”

“They're not gonna help you unless they're put on notice,” Mr Bannon said, suggesting that he would use his position at Breitbart to pressure Republicans to help push through Mr Trump’s agenda.