As the most successful British and American cities have gentrified and repopulated in recent decades, reversing the inner-city decline of the 60s and 70s, it’s become a cliche to say how powerful they are: economically, culturally, politically. Many people think they’re too powerful. A revolt against urban liberalism and multiculturalism, and their supposed imposition on the rest of the population, was a big element of the Brexit and Donald Trump campaigns.

Almost two-thirds of US rural and small-town voters chose Trump, while a similar proportion in the cities chose Hillary Clinton. In the English countryside, 55% voted for Brexit, while cities as varied as Bristol, Glasgow, Cardiff, Liverpool and London voted even more decisively for remain. The stark and growing political division of the US and the UK by population density has been one of the most striking, if under-reported, revelations of the great 2016 electoral reckoning.

Yet the US election and the EU referendum have also shown that even the most confident, expansive cities are politically quite weak. Not simply because their preferred causes lost narrowly in both cases; but because patterns of urban life and both countries’ electoral systems are increasingly out of sync.

According to the Office for National Statistics, there are currently more than a million non-British EU citizens living in London – almost an eighth of the city’s population. Much of the sense of present-day London as a teeming and important global city has come from this influx. Yet these converts to the capital, and other foreign residents from outside Ireland and the Commonwealth cannot vote in British general elections, only in local ones (assuming they have bothered to register during a stay that may be brief). Their presence may have a huge economic and social impact, but it has little political weight.

In the US, the revival of many cities has also left them under-represented. Between 1950 and 2015, the proportion of urban Americans rose from 64% to 82%. But the ancient, creaky workings of the electoral college mean that the recent population booms in downtown Los Angeles and Brooklyn, for example, have simply concentrated liberal voters even further in urbanised states that were already easy Democratic wins. Clinton’s futile popular vote victory suggests that hipsters are not a decisive electoral demographic, or at least not yet.

The very thing that makes modern cities vibrant and culturally dominant – increasing population density, and the atmosphere and networks that result from it – has left them politically under-represented. Meanwhile, the scattered and thinned-out populations of many struggling rural and small town areas distribute their voters through the British and American electoral systems much more efficiently.

A large and growing proportion of city Britons are not registered to vote at all. David Cameron’s government was as good at bending the pliable British political system in his party’s favour as it was bad at attracting urban support. Starting in 2014, it rushed the introduction of individual electoral registration, requiring voters to register themselves rather than letting others do so on their behalf. In a 2015 report, 10 Million Missing Voters!, the Smith Institute found that “inner-city areas, especially those with young and/or student populations and high levels of privately rented property” were “most at risk” of shrinking electoral registers.

Exactly that has happened. This June, in the left-leaning London borough where I live, the Hackney Citizen newspaper reported that in some local wards the proportion of eligible residents registered to vote was less than 70%, a fifth below the already unimpressive national average. A fortnight later, for the EU referendum, the Hackney turnout was only 65% – again, well below the national average of 72%. Put another way, less than half of eligible local residents had voted.

People living in cities are often transient, overcommitted, easily distracted. On the day of the referendum, psephologists expected a great urban voting surge for remain, especially in the evening, when liberal professionals got back from work. It never quite came. In the pro-remain strongholds Manchester and Glasgow, the turnout was even worse, at under 60%.

In the UK and the US now – and possibly France too, given the likelihood of an anti-metropolitan bidding war between François Fillon and Marine Le Pen – politics is dominated by voters who are less busy: country pensioners, unemployed or underemployed workers in ex-industrial areas. Instead of a city-style politics based on novelty and compromise – the approach of the Obama and Blair eras – Britain and American now have a politics that feels provincial, old-fashioned, almost millenarian, full of suspicion of outsiders and yearnings that somehow Brexit or a strongman president will make everything all right.

In the UK, the boundary changes to parliamentary constituencies will almost certainly marginalise cities further, as they will be based on the new electoral registration system. London, despite being in the middle of a population boom – much of it British – is expected to lose five of its 73 seats. The Conservatives insist that the rationale is to “reduce the cost of politics”, but it looks more like gerrymandering. Barely a third of London MPs are Tories.

What can liberal city-dwellers do about this? They can bother to vote. They can elect city leaders who make cosmopolitan urban values – what new Ukip leader Paul Nuttall cannily caricatures as “dinner party” thinking – appealing to rather than alienating voters elsewhere. Or they can wait for further urban population growth and the perpetual migration of city people and ideas to other places to shift the political balance.

Politics is never just about elections; it’s also about prevailing values and lifestyles, and who holds economic power. However triumphant and impregnable Trump, the Brexiteers and their anti-urban supporters seem now (and will their supremacy last once the consequences of their airy promises start materialising?), it is hard to imagine the big cities losing their enormous economic and cultural clout any time soon. The internet, for one thing, makes it impossible to keep urban ideas and behaviours corralled inside city walls.

In a democracy, electoral politics tends to adjust to social trends in the end. But the process can take a while. City liberals should hunker down: the city-haters rule for now.

• This article was amended on 14 December 2016 to clarify who is entitled to vote in British general elections.