THE Koch brothers are to the left what George Soros is to the right: villains accused of using their billions to subvert the will of the people. Charles Koch and his younger brother David are fabulously rich thanks to the success of Koch Industries, an oil, gas and commodities conglomerate based in Wichita, Kansas, that is America’s second-biggest private company and employs more than 120,000 people. Yet like Mr Soros, who made his billions as a hedge-fund manager and supports liberal causes on both sides of the Atlantic, they are better known for their political activism, which has made them heroes in conservative circles and the favourite bogeymen of liberals. On June 5th Charles announced in a letter to employees of Koch Industries that his brother was stepping down from his political and business interest because of poor health. David was diagnosed with prostate cancer over two decades ago and has battled the illness ever since. What will David’s retirement do to the conservative movement?

While the professorial Charles runs the family company out of Wichita, the more urbane David settled in New York and became the public face of the brothers’ political activities. He was the vice-presidential candidate of the Libertarian Party in 1980 and an outspoken (and munificent) supporter of Scott Walker, the Republican governor of Wisconsin, when he faced a recall election after a fight with public-sector unions. He is a driving force at the Cato Institute, a libertarian think-tank; Americans for Prosperity (AFP), a free-market group he started in 2004; Freedom Partners, a network of donors fostered by both Kochs; and other outfits promoting the libertarian credo. Together, they form what critics dub the “Kochtopus”. They will now have to do without him.

The brothers’ activism reached fever pitch during the presidency of Barack Obama, whom the Koch brothers considered a seriously misguided socialist. In a newsletter sent to his employees, Charles even compared the Obama administration to the regime of Hugo Chávez, Venezuela’s authoritarian leader. As they became the biggest backers of Tea Party organisations and helped to raise an estimated $400m to prevent the re-election of Mr Obama in 2012, the brothers transmogrified from libertarian outsiders to influential conservative power-brokers.

The price they paid for their often opaque activism was steep: an end to their privacy (until they took on the Obama administration, the Koch brothers were virtually unknown) and widespread vilification. Greenpeace published a report calling Koch Industries the “kingpin of climate-science denial”. An exposé in the New Yorker described how the Kochs gave millions of dollars to non-profit groups that promote scepticism about climate change, criticise environmental regulation and welfare programmes and support minimal taxes for industry—ostensibly in the name of their libertarian ideals which, conveniently, aligned with their corporate self-interest. David Axelrod, Mr Obama’s chief political strategist, singled out the Kochs in an op-ed in the Washington Post as billionaire oilmen secretly funding the Tea Party while pretending it was a grassroots movement for change. The Kochs received death threats that require the brothers to travel with security guards. “They became the caricature of capitalists,” says Daniel Schulman, author of “Sons of Wichita”, a book on the Koch clan.

In the past couple of years the Kochs have toned down their activism, in part because they could not bring themselves to back Donald Trump’s run for the presidency. (Charles once likened the choice between Mr Trump and Hillary Clinton to choosing between cancer and a heart attack.) They differ from Mr Trump on trade and immigration, explains Grover Norquist of Americans for Tax Reform, a lobby group. Three organisations financed by the Kochs, including AFP and Freedom Partners, announced on June 4th the launch of a multimillion-dollar campaign against Mr Trump’s tariffs on imports.

The Kochs’ political organisations, as well as the professorships they finance, will endure even without David’s active involvement, as long as the funds are flowing. They are run by seasoned political operatives who have become more eclectic in their choices. AFP recently paid for an advertisement thanking Heidi Heitkamp, a senator from North Dakota who is one of the most vulnerable Democrats up for re-election, for helping to pass legislation loosening some of the key provisions of Dodd-Frank, a stringent set of banking rules. And the Charles Koch Institute, which does public-policy research, is now focused on criminal-justice reform to fight what Charles calls the “overcriminalisation of America”. It is a cause espoused by liberal heroes such as Cory Booker, a senator from New Jersey—and by Mr Soros.