By endowing tax-beneficial private foundations, then disseminating funds through a complex web of charitable entities, 'a small, rarefied group of hugely wealthy, archconservative families' weaponised philanthropy, 'pour[ing] money, often with little public disclosure, into influencing how Americans thought and voted'. While dressed up as charity, the absolutist free-market, anti-government positions advanced by this 'dark money' benefited donors' 'bottom lines first and foremost'.

Mayer identifies the most sophisticated players as Charles and David Koch, multi-billionaire industrialists whose fortune was reportedly built on their father's refinery deals with Stalin and the Third Reich. Steeped in fringe libertarian theory, the Kochs considered politicians 'merely actors' and were determined to supply the 'themes and words for the scripts'. From the 1980s the Kochs built the political equivalent of a venture capital fund. They began with intellectuals, funding university centres, chairs, and grants that masked their true intent behind benign-sounding program names. By 2015, 'the Charles Koch Foundation was subsidising pro-business, antiregulatory, and anti-tax programs in 307 different institutions of higher education' sometimes requiring students' 'ideological improvement' to be tested. Research projects financed included 'why bank deregulation is good for the poor' and that 'mine safety and clean water regulations only hurt workers'.

To influence policy and national discourse, the network funded think tanks, augmenting this with bought media. Millions were spent for conservative radio hosts like Rush Limbaugh to push specific anti-government, pro-business ideas. Lawyers and lobbyists were funded to press cases in courts and Congress.

To give the illusion that these policies reflected the concerns of ordinary citizens, the Koch network deployed 'Astroturfs', 'synthetic grassroots groups'. At times these were wholly concocted – fake constituents' letters, agitators planted at town halls. But with the Tea Party, Mayer argues, an existing groundswell was co-opted. While the left's Occupy movement dissipated, the right's populist anger was 'funded, stirred, and organized by experienced political elites', then used to advance the policies of the ultra-rich.

Charles Koch (Wikimedia Commons)Dark money came to the fore with Obama's election in 2008 – seen as an existential threat to billionaires' interests. Within forty-eight hours of his taking office, Obama's (centrist, bipartisan) legislation to address the unfolding financial crisis was attacked: 'Think tanks funded by the Kochs and their allied network of donors, such as the Cato Institute, the Heritage Foundation, and the Hoover Institution ... began cranking out research papers, press releases, and op-ed columns opposing Obama's stimulus plan ... The paid advocates formed a national echo chamber' on Fox News and Limbaugh.

Cap and trade, a 'market-based solution' to address climate change 'originally backed by Republicans', threatened the Koch donors' major industry: fossil fuels. Notwithstanding that 'as late as 2003, seventy-five per cent of Republicans supported strict environmental regulations', Mayer shows how a 'private network waged a permanent campaign to undermine Americans' faith in climate science', spending 'over half a billion dollars' on misinformation and attacks on non-partisan scientists.

Nevertheless, the dark money was so well disguised that as late as 2010 Obama's team barely knew it existed. But after Citizens United removed restrictions on corporate spending in elections, dark money boomed. The 2010 congressional elections saw outside spending increase from two per cent in 2006 to forty per cent. By 2012, the Koch network's expenditure was $407 million, more than the total donations of 5,667,658 American voters. Targeted spending in 2010 congressional races destroyed unprepared moderate candidates (one described it as like being hit by a house) and elected extremists. With the Kochs' network now so well-funded and sophisticated that it began 'to supplant the Republican party', new GOP representatives were beholden more to their funders' narrow, anti-government interests.

Jane MayerCongress moved as hard and fast to the right as it has in recorded history. Narrow ideological interests repeatedly trumped the nation's. Mayer shows how this resulted in the ousting of then-Speaker John Boehner for being insufficiently conservative, and examines the extremist tactics used in two disastrous budget fights. One courted the US Treasury's first-ever default, and sacrificed the nation's AAA rating, to preserve tax loopholes. The other shut down the government to try and defund Obamacare – a law passed by both houses, upheld by the Supreme Court, and given a mandate by Obama's 2012 re-election.

The net result of this decades-long campaign has been to coarsen American political discourse, inflaming hyper-partisan opposition where there was previously bipartisan support, and rendering equal participation in democracy a quaint anachronism. Mayer's exceptional book reveals the money trail behind this pervasive threat to democracy. It remains to be seen in November just how far down the rabbit hole America will fall.