Bernie Sanders has raised the alarm about voting problems in the Michigan Democratic primary, demanding the state’s authorities make sure there are no obstacles to voting, and pointing to nationwide efforts at voter suppression.

Mr Sanders tweeted his concern: “At a time when Democrats correctly attack Republicans for voter suppression, to see voters standing in long lines for hours in Michigan and around America is an outrage.

“Election officials must address these problems immediately, and if necessary, keep polling places open longer.”

Mr Sanders’s state political co-ordinator, Abshir Omar, reported that one student voting at Michigan State University had waited for more than an hour to vote before giving up, saying “this is bulls**t, I can’t wait this long, I have to get to work”.

Given the Democrats badly need to win Michigan in order to beat Donald Trump, the spectre of voting problems there has them deeply concerned.

Just days before the 2016 election, the Michigan Democratic Party sued the Trump campaign, claiming it was engaged in efforts to intimidate and obstruct white liberals, young women and black voters. Similar lawsuits were filed in Arizona, Nevada, Ohio and Pennsylvania.

Legal battles over voter access in Michigan continue to this day. One current battle in the state focuses on Republican-backed measures restricting organisers from helping people fill in absentee ballots or using a vehicle to transport them to the polls unless they cannot walk.

Another state that has spent years making it harder to vote is Wisconsin, another crucial 2020 battlefield. Starting in 2010 under Republican governor Scott Walker, Wisconsin not only passed tough voter ID laws but restricted early voting hours, gerrymandered district boundaries in favour of Republicans and made it harder to register to vote.

As with similar moves in other states, the measures have been criticised for targeting groups that tend to cast Democratic votes. In 2016, Hillary Clinton lost the state to Mr Trump by only 22,748 votes — less than 1 per cent.

However, in good news for Democrats, a recent effort that would have thrown nearly 300,000 people off the state’s voter rolls was recently overturned in the Wisconsin Appeals Court.

The Democratic primary has seen serious pressure on the voting process in other states besides. In Texas, some voters were forced to wait in line for as long as five hours on Super Tuesday. Black and Latino neighbourhoods were disproportionately affected.

This is part of a pattern that’s playing out across the country. Democrats have for years been trying to expand the pool of swing states to include former Republican strongholds like Texas, Arizona and even Georgia. They are doing this by appealing to and organising the demographics where they do best — in particular with young people and black and Latino voters.

Texas, still run by Republicans at the state level, is just one of several states to have reduced the number of polling stations and introduced strict voter ID laws. These measures have proliferated since a 2013 Supreme Court decision struck down a key provision of the Voting Rights Act that forced jurisdictions with a history of discrimination to seek approval before changing their laws.

Among the states to have taken advantage of their new freedoms following the decision are several Republican-governed southern states with many black voters who tilt towards the Democrats.

One law in North Carolina, which has lately been considered a swing state, was blocked by the Supreme Court in 2017 after a federal appeals court described it as an effort to “target African-Americans with almost surgical precision”.