The attack on voting rights unleashed by Republican lawmakers over the past three years has made casting a ballot in parts of the deep south as fraught as it was in 1965 before the Voting Rights Act banned racial discrimination in elections, electoral monitors say.

Marion Warren, the registrar of voters for the small town of Sparta, Georgia, said that officials in local Hancock County have been so ruthless in impeding voting by the black community that the clock has been set back 50 years. “It’s harder for a minority to vote now than it was in the state of Georgia in 1965 – it’s causing voter apathy all across the county and that’s the best form of voter suppression you can find,” he said.

Warren was making his bleak assessment on the third anniversary of Shelby County v Holder, the controversial ruling by the US supreme court that punched a gaping hole in the Voting Rights Act that for half a century had assured minority groups of untrammeled access to the polls. Decided precisely three years ago, on 25 June 2013, the ruling put an end to safeguards that had obliged the worst offenders – mainly states or parts of states in the deep south – to apply for federal approval before they tampered with any aspect of their voting procedures.

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The minute the court’s opinion was unveiled, an unseemly scramble began by Republican-controlled state assemblies to erect new barriers to voting that disproportionately affected African American, Latino, Native American, older and poor voters. Texas rushed through its voter ID law within hours of the supreme court announcement. North Carolina passed its monster voter suppression law, HB589, less than a month after Shelby.

The upcoming presidential election will be the first big test of the new post-Shelby, and thus post-Voting Rights Act, world. The prospects are ominous.

The Brennan Center for Justice has calculated that come November some 17 states will have new voting restrictions in place for the first time in a presidential race. Of those, only seven had been covered by the federal “pre-clearance” rules, suggesting that Shelby has had a wide knock-on effect beyond states immediately impacted.

“The decision has certainly emboldened states, even those that weren’t covered, to feel that it’s OK to go ahead and put in place ruses that will make it hard for people to vote,” said Jennifer Clark, a Brennan Center counsel specializing in voting.

In Georgia, the state has attempted to put in place almost 20 discriminatory changes in the past three years, according to the NAACP Legal Defense and Educational Fund. They include redistricting to dilute the electoral power of black voters, cuts to early voting and moving early voting locations into police stations.

In Sparta, the mainly white Hancock County board of elections last year carried out a so-called purge of the voter rolls in the town. One in five registered voters in the town, which is overwhelmingly African American and poor with per capita income of $11,400 a year, were challenged under the purge.

Fifty-three people – a sizeable number in a town of just 1,500 – were summarily taken off the rolls and hence disenfranchised. Some of those have been reinstated through the intervention of federal courts.

Warren said that he also had to deal with other attacks on the voting rights of his townspeople. “They tried to close all the polling places in the county except for four out of the 10, which would force some people to travel 35 miles just to have their democratic say.”

A new report from the Leadership Conference Education Fund titled Warning Signs chronicles the various forms of skullduggery and sleight of hand that have been practiced by officials in Georgia as well as Arizona, Florida, North Carolina and Virginia. It shows how in states where the presidential battle between Donald Trump and Hillary Clinton is tight, the mere impediments put in the way of largely Democratic-leaning minority groups could swing the election.

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In Florida, for instance, a key swing state that has tended to produce very close presidential results, there are more than 1.5 million formerly incarcerated individuals who have been stripped of their right to vote even though they have completed their sentence. In elections held in October 2013, Monroe County, which used to be covered by the Voting Rights Act, conducted an election entirely in English, with no Spanish translation on any ballots or electoral materials despite its large Latino population – precisely the hurdle to voting that had brought Monroe under federal supervision under the act in 1975.

In Virginia, estimates suggest that almost 200,000 registered voters do not have acceptable ID cards to show at polling stations under the state’s new photo ID law. That vastly supersedes the margin of just 17,000 votes that decided the outcome of its 2014 US Senate election.

In Texas, the number of people who could be caught out by lacking an accepted photo-ID card could be as high as 600,000.

Overall, the national racial justice group, the Advancement Project, calculates that 25% of African American citizens of voting age do not have a current government-issue photo ID card, and 16% of Latino citizens. That contrasts with 8% of white citizens.

Wade Henderson, president of the Leadership Conference, said that across the five states covered by the new report some 84 presidential electoral college votes were at stake, as well as two senate seats and one governor’s mansion. He said the spate of measures amounted to a “post-Shelby renaissance in voter discrimination”.

“With the presidency and control of the Senate at stake, the temptation to shave points off of the participation rate of voters of color has proven too great to resist. Since Shelby, states have engaged in deceptive and sophisticated efforts to disenfranchise voters that will have an impact on the 2016 election.”