(Reuters) - A federal judge on Wednesday ruled Michigan cannot ban "straight-ticket" voting, allowing voters to choose all a party's candidates with just one bubble on a ballot, saying the law prohibiting the practice was racially discriminatory.

The ruling permanently blocks what U.S. District Judge Gershwin Drain called a politically motivated move by the Republican-controlled state legislature in a state that backed President Donald Trump in 2016 after twice choosing Democratic former President Barack Obama.

The ban did not affect the November 2016 election as a temporary order had blocked the state from enforcing it.

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Drain cited research finding African-American voters are more likely than voters of other races to cast a straight-ticket ballot and are more likely to vote Democratic than Republican.

"The goal of ending the Democratic Party's success with straight-ticket voters, therefore, was achieved at the expense of African-Americans' access to the ballot," Drain wrote in a 103-page ruling.

A spokesman for Michigan Secretary of State Ruth Johnson, named as a defendant in the lawsuit, declined immediate comment on the merits of the decision.

Voter access has become a politically charged issue, with Democrats accusing Republicans of trying to suppress minority votes by making it more difficult to register to vote or to cast ballots. Republicans defend the moves as necessary to prevent the casting of fraudulent ballots.

Michigan will be an important contest in the November midterm elections, where Democrats aim to flip 23 seats in the U.S. House and one in the Senate to recapture the majority and have a chance at blocking Trump's agenda. One of the state's Democratic U.S. Senators, Debbie Stabenow, is up for reelection this year.

Michigan is one of just nine states where voters are allowed to pick all a party's candidates for office with just one ballot mark. No states prohibit voters from individually choosing all candidates from one party.

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State Republican Senate Majority Leader Arlan Meekhof said he was "disappointed" in the ruling.

"The straight-ticket option is antiquated and does not reflect the independent nature of most voters," Meekhof spokeswoman Amber McCann told the Detroit News on Wednesday. McCann could not be reached for immediate comment.

Government watchdog group Common Cause called the ruling "a big win for Michigan voters."

"Judge Drain's decision makes clear that the Michigan legislature intentionally discriminated against African Americans for partisan political purposes," the group's president, Karen Hobert Flynn, said in a statement.