Hillary Clinton may have been the Democratic victor in Iowa last Monday, but the scale of Bernie Sanders’s achievement there was as significant as his win in New Hampshire last night. Clinton is not only a former secretary of state and first lady, but has vast resources at her disposal. Before Christmas, George Soros donated $6 million to her Super PAC, Priorities USA. That’s the equivalent of 228,000 Sanders supporters each donating $26.28, the average contribution his campaign has received so far.

The Vermont senator who describes himself as a ‘democratic socialist’ was able to take America’s best-connected politician to a ‘virtual tie’ in Iowa, and defeat her in New Hampshire, because his campaign has galvanised the young more than anyone could have expected. The only reason Iowa was a race at all – as Eric Levitz elegantly put it – was because, among the young, it wasn't.

According to the Iowa entrance poll, Sanders beat Clinton by almost 6-1 among the under-30s, taking 84 per cent of votes compared to Clinton’s 14 per cent. Sanders also enjoyed a 20 point lead among 30 to 44-year-olds, but Clinton was peerless among the over-45s, and won 70 per cent of the vote among over-65s.

The Democrats have won more of the popular vote than the Republicans in five of the last six presidential elections, and with the country’s changing ethnic composition – according to the US Census Bureau, 'the US is projected to become a majority-minority nation for the first time in 2043' – the pattern looks set to continue. People of colour are more likely to vote Democrat, and demographic shifts could keep the Republicans out of the White House for more than a generation. The Republican establishment, aware of the gravity of the problem, has tried to address it for the best part of a decade, as evidenced in the favoured candidacy of Marco Rubio and Jeb Bush's mentioning his Mexican wife whenever he can. So far, however, it's failing, and the rhetoric of Donald Trump and Ted Cruz on immigration is likely to have undone any good will from minorities. Simply put, much of the Republican core vote is at odds with the direction the party has to take in order to reflect a changing electorate.

The transformation in American politics since George W. Bush won a second term in 2004 is remarkable. Changing social attitudes on a range of issues, from race to gay rights, are remaking the country. Non-medical use of cannabis has been decriminalised in several states, and same-sex marriage has been legal nationwide since last June. Barack Obama arrived in office heralded as a liberal, but he has been repeatedly outflanked by movements to his left, in particular on income inequality and police violence against people of colour. The most important social movement of his presidency wasn’t the Tea Party; it was Black Lives Matter. Ask anyone who watched the half-time break at Super Bowl 50.

This evolution, both within and beyond electoral politics, is being driven not only by changes in the country’s ethnic composition. A large birth cohort – ‘millennials’ – now outnumber baby boomers. Aged 18 to 34, this racially diverse, economically stressed and politically liberal generation is now the largest in the US labour force. Slowly, they are redefining its politics. The success of Bernie Sanders, even if he doesn't win the Democratic nomination, is a political breakthrough for them. It is unsurprising that, as in Britain with Jeremy Corbyn, they are represented by an older politician (Corbyn is 66; Sanders 74). After the military interventions in Iraq and Afghanistan, a generation lost faith in party politics, the potential leaders among them more inclined towards direct action than electioneering.

Sanders may not win the Democrat nomination – although the data from his win in New Hampshire last night is incredibly promising – just as Labour may not win the 2020 general election. Yet both Corbyn and Sanders appeal to a new normal among many middle-aged and younger voters: high levels of debt, little in the way of asset ownership, an uncertain future and a sense that any historical social contract has long been hollowed out. Critics of Corbyn, unwilling to look at the data on who actually voted for him, call his thinking antiquated and out-of-touch. They couldn’t be more wrong. The politics of Bernie Sanders and Jeremy Corbyn is the leading edge of something far bigger than either man, or even their respective parties. A generational shift, regarding the role of government and much else besides, is under way. At some point in the next decade it will be the bedrock for a different kind of consensus from the one that currently prevails.