Almost 6 million Americans will not be able to vote in November's presidential election under tough state rules that have pushed the number of disenfranchised former convicted criminals to a historic high.A new study by the Sentencing Project estimates that a record 5.85 million people – some 2.5% of the US voting age population equivalent to one out of every 40 adult Americans – will be ineligible to vote in November by dint of having been convicted of a felony. That includes almost 3 million people who have served their sentence in full, including all probation, and yet are still stripped of their right to vote under harsh state laws.

The US is the among the strictest nations in the world in terms of denying the vote to those who have felony convictions on their record. The Sentencing Project report shows how the laws have been sharply toughened up in recent years across many states, dramatically increasing the numbers caught in the felony trap – from just 1.2 million people in 1976 to 5.9 million in 2010.

African Americans and other minority ethnic groups are particularly vulnerable to being disenfranchised. Almost 8% of adult African Americans are ineligible to vote because of convictions, compared to 1.8% of the rest of the adult population.

In three states with the harshest laws – Florida, Kentucky and Virginia – more than one in five black Americans have been stripped of their vote.

The specifics of the voting rights of current and former convicts is left to the discretion of individual states in America, and practice varies widely. Maine and Vermont, both bastions of liberal penal approaches, allow current inmates of prisons and detention centers to vote.

By contrast, 11 states, mainly concentrated in the south and south-west of the country, deny voting rights to some or all of the "ex-felons" who have paid the price of their crimes in full, successfully passing through prison, parole and probation.

Florida holds the dubious distinction of being the nation's capital for disenfranchisement. According to the Sentencing Project, the state has stripped more than 1.5 million people of the vote because of their criminal records – about one in 10 of all adult Floridians.

That is a statistic that potentially has huge electoral significance. Florida is among the most hotly contested of states in a presidential election year, and African Americans tend to vote heavily in favour of Democratic presidential candidates.

In 2008 Barack Obama took the state with fewer than 250,000 votes over his rival John McCain. George Bush notoriously won the state, and hence the presidency, in 2000 with just an official margin over Al Gore of 537 votes.

© Guardian News and Media 2012

[Man behind bars via Fer Gregory / Shutterstock]