Following reports that voters were leaving long lines in Michigan after waiting hours to cast their ballots in the state's primary election, Bernie Sanders called the delays an "outrage" and urged election officials to consider leaving the polls open longer.
In a statement, he said: "At a time when Democrats correctly attack Republicans for voter suppression, it is disappointing to see people standing in long lines for hours today waiting to vote in Michigan and around the country. People should not have to miss a day of work to exercise their right to vote."
On Twitter, Mr Sanders' political director in Michigan shared a story from a Michigan State University student who left his voting queue after waiting for an hour to vote.
Abshir Omar said: "A Michigan State University student after waiting over an hour in line to vote, called it quits and walked out. He said 'this is bulls***, I can't wait this long, I have to get to work."
He directed his message to the university, saying: "This is unacceptable."
On Tuesday, Michigan and five other states cast their ballots for the 2020 Democratic presidential nomination as the Vermont senator and former vice president Joe Biden emerge as the remaining front-runners.
Long lines formed at clerks' offices across the state, which allows same-day registration at the polls, as clerks fielded hundreds of requests for absentee ballots, the results of which will likely postpone the final results.
Reports of long wait times on Tuesday followed several hours-long lines to vote across Texas during Super Tuesday elections on 3 March.
That state had closed hundreds of polling locations over the last eight years, a majority of which had closed following a US Supreme Court decision that struck down a portion of the Voting Rights Act requiring states with a history of racial discrimination at the polls need to seek federal approval before making changes to their election processes.
That decision "catalysed a systemic examination of poll closures and other seemingly innocuous changes that could have negatively impacted voters of colour" who tend to vote Democratic, according to a report from The Leadership Conference on Civil & Human Rights.
