Democracy is, by definition, people power. But even now the question of who constitutes “the people” remains. The answer may determine whether Donald Trump remains president after 2020. Democracy in America faces many perils, from dark money to foreign interference, but one goes directly to its central promise of one person, one vote.

Attempts to restrict the right to vote are as old as the struggle to expand it beyond wealthy white men. In the US, it took the civil rights movement to extend the franchise to all African Americans. Efforts to erode it have intensified in the last two decades – particularly since 2013, when a supreme court ruling gutted the 1965 Voting Rights Act, which required federal approval of changes to election laws and policies.

As a year-long Guardian series, The Fight to Vote, sets out, more than half of US states have since passed laws suppressing the votes of the poor, the young and the non-white – all groups more likely to vote Democrat. In North Carolina, a federal appeals court noted, the Republican legislature requested data on the use, by race, of voting practices – then implemented changes that “target[ed] African Americans with almost surgical precision”. Although such efforts are most associated with the south, Ohio and Wisconsin have also passed strict voter-identification or roll-purge acts. (Gerrymandering can be regarded as another denial of voting rights, since it removes the power of a person’s ballot.)

The blatant tactics of earlier times, such as literacy tests, have been replaced by more insidious means. Closures of polling sites, restrictions on polling hours and early voting disproportionately affect black people, thanks to their employment patterns. Measures come cloaked in bureaucratic or even righteous guise: absurdly restrictive rules on voter identification are presented as an attempt to tackle the phantom threat of fraud. Attempts to challenge and overcome such laws are themselves criminalised, with registration drives in states such as Tennessee treated as suspicious. Meanwhile Republicans snub election security legislation to address proven Russian meddling.

In 2016, black voter turnout fell by 7 percentage points. Barack Obama had driven record turnout in 2008 and 2012; but this was also the first presidential election for half a century without key protections. In Georgia last year, Republican Brian Kemp beat the black Democratic candidate Stacey Abrams to the governorship by just 50,000 votes. As its secretary of state, with oversight of elections, he had purged 1.5 million voters from the rolls.

Last year’s midterms raised the stakes further. Republicans understand what they are up against electorally. This spring the Democrat-controlled House passed a sweeping package of ethical and election reforms including the establishment of automatic national voter registration. Mitch McConnell, the majority leader of the Republican-controlled Senate, wouldn’t allow a vote on it. And a second term would allow the Trump administration to keep stacking federal courts with rightwing judges, and exacerbate the supreme court’s rightwards tilt.

The UK also needs to pay attention. The government wants to introduce compulsory photo identification for voters – a move that campaigners warn would disenfranchise tens of thousands of people. But if the US offers a warning, it also offers signs of hope. Carol Anderson, author of One Person, No Vote, cites the role of a voting registration drive in Alabama last year in speeding Doug Jones to victory over Roy Moore in the Senate race. Stacey Abrams is leading a nationwide campaign against voter suppression. Efforts to restrict voting rights will redouble. But resistance is growing too.