WASHINGTON (Reuters) - The U.S. Supreme Court on Friday rejected a bid by Michigan to reinstate its Republican-backed ban on straight-ticket voting for the Nov. 8 general election.   The justices left in place a decision by a federal district judge in Michigan who in July suspended a law that abolished straight-ticket voting, the practice of using one mark to vote for all candidates from one party, finding that it would disproportionately affect black voters.  The 6th U.S. Court of Appeals upheld that finding last month, prompting the state to seek a stay from the Supreme Court. Two conservative justices on the eight-member court, Clarence Thomas and Samuel Alito, voted to grant the request, the brief order said.    The Michigan law, passed by a Republican-controlled legislature and signed by a Republican governor, was one of numerous voting measures passed at the state level that put new restrictions on voting. These measures also include stricter voter-identification laws and reduction of early-voting periods before election day. Proponents of the law, enacted in January, have said most states have moved away from a straight-ticket voting option. Removing the option forces voters to study candidates and encourages voters to make decisions based on criteria other than party affiliation, they said. Opponents say voting restrictions are aimed at reducing turnout of minorities, who are more likely to vote for Democrats. U.S. District Judge Gershwin Drain in July granted a preliminary injunction sought by civil rights and labor groups who sued Michigan’s Secretary of State Ruth Johnson and Attorney General Bill Schuette. The judge said elimination of straight-ticket voting would be a burden on voting rights and cause long wait times at polls. After Friday’s decision, Schuette said, “It is my duty to defend Michigan’s laws, in this case a law that stands in 40 other states. Now the U.S. Supreme Court has spoken and I will respect that decision.” The case is one of several voting disputes being litigated ahead of the election and is the second emergency application the Supreme Court has recently been asked to handle. On Aug. 31, the court rejected a bid by North Carolina to reinstate for November’s elections several voting restrictions, including a requirement that people show identification at the polls. The high court is short one justice following the death of conservative Antonin Scalia in February. As a result the court is evenly split 4-4 between liberals and conservatives. 