Donald Trump has launched an extraordinary attack on the Koch brothers, accusing the Republican megadonors of opposing his government’s agenda.

“The globalist Koch Brothers, who have become a total joke in real Republican circles, are against Strong Borders and Powerful Trade,” the US president wrote on Twitter early on Tuesday morning.

“I never sought their support because I don’t need their money or bad ideas.”

Mr Trump’s outburst came after the Koch brothers’ political arm declared it would not help elect a Republican senate candidate in North Dakota, partly over his failure to challenge the White House’s trade tariffs.

The decision sent a strong message to Republican officials across the country unwilling to oppose the spending explosion and protectionist trade policies embraced by Mr Trump.

“For those who stand in the way, we don’t pull any punches, regardless of party,” Tim Phillips, who leads the Kochs’ political arm Americans For Prosperity (AFP), told hundreds of donors during a three-day private Rocky Mountain retreat.

But a furious Mr Trump hit back, claiming he made Charles and David Koch “richer”, and that they “love” his tax cuts, deregulation and judicial nominations.

“Their network is highly overrated, I have beaten them at every turn,” he continued. “They want to protect their companies outside the US from being taxed, I’m for America First & the American Worker – a puppet for no one. Two nice guys with bad ideas. Make America Great Again!”

The split marks a new chapter in the strained relationship between the Trump administration and the expanding conservative network created by billionaire industrialists, who refused to endorse the Republican president in 2016.

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Mr Trump has effectively taken over the Republican Party on almost every level, even after ignoring long-held conservative beliefs on government spending, free trade and foreign policy. The billionaire Kochs and their nationwide army of conservative activists, however, are not giving in.

That is not to say they are punishing every Trump loyalist in the 2018 election season.

AFP still plans to focus its resources on helping Republican senate candidates in Tennessee, Florida and Wisconsin. It remains unclear how hard the group will work to defeat vulnerable senate Democrats in West Virginia, Missouri and Montana.

The midterm strategy could change in the coming weeks, but the Kochs currently plan to ignore North Dakota’s high-profile senate contest, where three-term Republican congressman Kevin Cramer is trying to unseat Democratic senator Heidi Heitkamp. She is considered among the most vulnerable senate Democrats in the nation.

“He’s not leading on the issues this country needs leadership most right now,” Mr Phillips said of Mr Cramer, specifically citing spending and trade. “If Cramer doesn’t step up to lead, that makes it hard to support him.”

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Ahead of the announcement, Charles Koch told reporters that he cared little for party affiliation and regretted supporting some Republicans in the past who only paid lip service to conservative principles.

Network leaders over the weekend repeatedly lashed out at the Republican-backed $1.3 trillion (£990bn) spending bill adopted in March, which represented the largest government spending plan in history. The Trump White House budget office now predicts that next year’s federal deficit will exceed $1 trillion, while reaching a combined $8 trillion over the next 10 years.

The Kochs were equally concerned about the Trump administration’s “protectionist” trade policies, which have sparked an international trade war and could trigger a US recession, Charles Koch said.

“We’re going to be much stricter if they say they’re for the principles we espouse and then they aren’t,” he vowed. “We’re going to more directly deal with that and hold people responsible for their commitments.”

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The Koch network has demonstrated in recent months – albeit on a limited basis – a willingness to praise Democrats and condemn Republicans in specific situations.

After first running attack ads against Ms Heitkamp earlier in the year, the Kochs last month launched a digital ad campaign thanking the North Dakota Democrat for voting to roll back Obama-era banking regulations. At around the same time, they launched an advertising blitz to criticise 10 Republican House members, including Pennsylvania Republican senate nominee Lou Barletta, for supporting the massive spending bill.

Following Monday’s announcement, Julia Krieger, a campaign spokesperson for Ms Heitkamp, said, “When it comes to leading on the pocketbook issues North Dakotans care about – from strong trade markets to responsible spending and cutting red tape for North Dakota businesses – Heidi has always been consistent: North Dakota comes first.”

The development marked a dramatic escalation in the Kochs’ willingness to buck partisan loyalties. And some Trump loyalists were furious with the Kochs’ work to undermine Trump and his agenda even before Monday’s news dropped.

Former White House adviser Steve Bannon questioned the true influence of “the Koch network management”, seizing on the lack of accountability in the organisations’ spending in recent years given that most of the details are not publicly available.

“Where did the money go, what do they really spend it on, and how much, if anything, do they really put into the network?” Mr Bannon asked in a brief interview with The Associated Press.

And prominent Texas-based Trump donor Doug Deason, who attended the weekend retreat, said Republican candidates should not be punished for embracing the president’s agenda.

“That’s not right,” he said before Monday’s announcement, condemning the Koch network’s recent decision to praise Ms Heitkamp.

“Heitkamp, we’re going to knock her out of the water. She’s gone,” Mr Deason predicted.

The decision to ignore the Republican candidate in North Dakota certainly caught some by surprise, but there appeared to be overwhelming support from others – even if the plan hurts the GOP’s push to maintain its House and Senate majorities.

Kentucky governor Matt Bevin, among a handful of elected officials who mingled with donors at the weekend retreat, said there should be political consequences for those who deviate from conservative principles.

“If in fact you have people espousing these in name, but not in practice, yeah, they’re not going to be supported, nor should they be,” Mr Bevin said in a brief interview. “I think this network supports people who truly respect those principles. And I think they’re agnostic, from what I’ve seen, with respect to what party a person is.”

At the same time, Mr Bevin defended Mr Trump’s push to apply billions of dollars in tariffs on goods from China, Canada, Mexico and the European Union. He dismissed the outcry from businesses in Kentucky and elsewhere as a short-term problem.

Colorado-based energy investor Chris Wright, a longtime Koch donor, said the Republican Party may have lost its way in the age of Mr Trump. He and his wife, Liz, encouraged the Koch network to ignore Republican candidates who turn their back on key conservative principles out of loyalty to Mr Trump.

“They don’t deserve to be funded if they don’t uphold our values,” Liz Wright said.