The Vermont senator tells the Guardian why the tax bill – which could become law next week – is the result of 40 years’ scheming by the Kochs and others

Bernie Sanders has accused Donald Trump and Republican party leaders of capitulating to the demands of the Koch brothers and other wealthy rightwing donors in railroading $1.5tn tax cuts through Congress.

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In an interview with the Guardian, the independent US senator from Vermont denounced the tax bill as payback for the billions of dollars that donors have invested in conservative politicians over decades. “What this is all about is nothing more than the Republican party very generously rewarding their wealthy campaign contributors,” he said.

The former presidential challenger said the reform of the tax code, which could clear both the House and the Senate and be signed into law by the the US president as early as next week, was based on the “rightwing extremist ideology of the Koch brothers. You can read it in what they were saying 40 years ago. What they want is an oligarchic form of society in which government plays virtually no role in public education, healthcare or addressing the needs of middle-class and working families.”

The end game, Sanders said, was that “you are on your own. You are 80 years old and you have cancer – good luck to you. Government is not there for you.”

The legislation, which has now been merged to form a single version from earlier Senate and House bills, marks the most substantial redrawing of taxation in America since Ronald Reagan’s 1986 tax cuts. It would slash the tax burden on corporations from 35% to 21%, and the top level of individual taxes from 39.6% to 37%.

The tax package is just another Trump lie. Many millions of middle-class Americans will pay more in taxes Bernie Sanders

But while the bonus to corporations is permanently set in stone, benefits to working families would expire after a number of years. That would leave millions of middle-class families – 83 million, Sanders said – eventually paying more in taxes under the changes, while the most wealthy Americans would continue to cash in.

“This is nothing more than an effort to make the very rich richer,” Sanders told the Guardian. “It is based on the fraudulent theory of trickle-down economics that never worked, never will work.”

Sanders warned of a double blow to working people in which the vast increase in the deficit caused by the tax cuts would subsequently be used by Republicans to justify an assault on welfare benefits. Paul Ryan, the GOP Speaker of the House, has announced that he intends to turn to healthcare and anti-poverty programs next year to reduce spending, and conservatives continue to harbor ambitions of destroying Barack Obama’s signature legislation, the Affordable Care Act, also known as Obamacare.

Facebook Twitter Pinterest Paul Ryan looks set to reduce welfare programs to limit spending. Photograph: Jim Lo Scalzo/EPA

Sanders said that “after running up a $1.4tn deficit over 10 years they are going to offset that deficit by making massive cuts to social security, Medicare and Medicaid. That is clearly the intention.”

The senator ran Hillary Clinton to a close second place in the 2016 race for the Democratic presidential nomination. Since then he has become arguably the most prominent critic of America’s growing income and wealth inequality and the plight of the working poor.

His attack on the Republicans’ tax plans came a day after the UN special rapporteur on extreme poverty and human rights, Philip Alston, issued a withering report that denounced growing in equality in the US. Alston’s initial findings, released on Friday, criticized the tax bill currently going through Congress for hugely increasing already large disparities between rich and poor.

Sanders’ claim that the tax cuts are a thinly disguised reward to billionaire extreme rightwing donors is supported by the statements of some Republican politicians themselves. Chris Collins, a New York congressman, told reporters that pressure from donors made the reform essential.

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“My donors are basically saying, ‘Get it done or don’t ever call me again,’” he said.

Last month the leader of the powerful conservative Super Pac the Congressional Leadership Fund, made it clear to the Washington Post that lawmakers who stood in the way of the tax cuts in the House should not expect to receive any of the $100m the body expected to spend on candidates in next year’s midterm elections.

David and Charles Koch have leveraged their own vast wealth – they own Koch Industries, the second largest private company in the US, and are valued by Forbes at $49bn each – by creating a network of other similarly minded wealthy conservatives that runs parallel to the Republican party. In the 2016 election cycle it spent about $250m on candidates who followed their anti-government creed, and the plan for next year is to increase that investment to $400m.

The Koch network has thrown its muscle behind the tax cuts this year. One of its periodic retreats for super-rich donors in New York in October was devoted to the subject of tax reform and attended by the vice-president, Mike Pence.

The brothers first entered the political arena in the 1970s when they began agitating for a bonfire of restrictions on spending by rich individuals and companies on political campaigns. They have consistently fought against government regulations and social programs helping low-income families, in ways that have been advantageous to their business interests that include oil and other energy concerns.

“Anyone who has looked at the Koch brothers and folks like them who have contributed over the years probably billions of dollars in right-wing efforts should not be surprised at the policies coming from Paul Ryan and Donald Trump,” Sanders said.

The senator lamented the unprecedented speed with which the tax plans have been rushed through Congress, and the consequent lack of public debate. As a result, he said, many details of the legislation were still so sketchy they were impossible to understand.

One area that Sanders is especially wary of in the unfolding bill are provisions that would benefit hedge fund managers based on tax havens such as the Virgin Islands. “We believe the provision will apply to probably fewer than 10, maybe even as few as three, hedge fund managers that will result in some $600m in tax breaks over 10 years.”

He added that he had raised the issue on the floor of the US Senate, asking for an explanation from the Republican group, “but we have not been given one”.

Sanders said the tax cuts were particularly punishing for those working Americans who had voted for Trump last November partly on the expectation that his promised tax cuts would ease their financial burdens.

“The tax package is just another Trump lie. Many millions of middle-class Americans will pay more in taxes and if Republicans get their way – and we are going to fight them tooth and nail on this – there will be cuts to social programs that many of Trump’s supporters have depended on for years.”