The Supreme Court on Friday refused to let Michigan enforce a ban on casting straight-ticket ballots in the coming election after lower courts found the prohibition was likely to discriminate against African Americans and result in long lines at the polls.

The justices declined to get involved in a controversy that began when the state’s Republican leadership passed a bill to end 125 years of straight-ticket voting, which allows a voter to choose all the candidates of a desired party by casting a single vote.

The Supreme Court gave no reason for its decision to turn down Michigan’s request that it be allowed to enforce the ban. But the decision was another sign that it will be difficult for those bringing election controversies to the court in advance of November to prevail.

Justices Clarence Thomas and Samuel A. Alito Jr. said they would have granted the state’s request but also did not reveal their reasoning.

Last week, the court deadlocked on North Carolina’s request to use a strict voting measure that an appeals court had found deliberately discriminated against African Americans. The law will not be in place for the coming elections.

In Michigan, voters have twice rejected attempts to abolish straight-ticket voting.

But the legislature in late 2015 did just that, joining 40 other states that do not allow the practice. The state said eliminating the option increased the chance that a voter would not overlook nonpartisan issues on the ballot and also would make a “more informed vote by examining the credentials and values of each candidate.”

But Democrats said the action by the state’s Republican leadership, including Gov. Rick Snyder, was partisan. The black labor organization, the A. Philip Randolph Institute, and Common Cause sued, saying the law was meant to discourage minority voters who overwhelmingly choose Democrats.

A district judge and then a panel of the U.S. Court of Appeals for the 6th Circuit agreed, saying the elimination of the procedure would slow the voting process and create long lines in precincts where African Americans are most likely to vote. They also held that the new state-approved ballot, which could be construed to make voters think straight-party voting was still allowed, was confusing.

The courts agreed the law would not likely survive constitutional challenge, might violate the Voting Rights Act and could not be used in the coming election.

The challengers presented “unrebutted evidence in the record demonstrating that [the law] will increase the time that it takes to vote, particularly in African-American communities where straight-party voting is prominent and where lines are often already long,” wrote Circuit Judge Karen Nelson Moore. “The district court also found that the law was likely to increase voter confusion and miscast ballots. Although this burden is not severe, it is also not slight. In the face of this burden, the state has offered only vague and largely unsupported justifications of fostering voter knowledge and engagement.”

The Michigan news organization MLive.com found in a survey that about half of Michigan voters in 2012 used the straight-ticket option. About 30 percent of voters chose the Democratic Party using the option, while a little under 20 percent chose Republicans, according to the survey.

Michigan Attorney General Bill Schuette, a Republican, had urged the Supreme Court not to allow lower courts to prevent the state from doing what other states have done.

“Having voters actually cast a vote for their chosen candidate — rather than blindly voting for all candidates of a party — is the very act of voting, so it cannot rationally be characterized as a burden on the right to vote,” he wrote in his petition to the court.

The challengers said the suit was not claiming a constitutional right to straight-ticket voting. “It is about the unconstitutional consequences for millions of voters of eliminating this option in the unique context of Michigan elections,” they told the court.