DETROIT (AP) — A judge on Thursday blocked Michigan's new ban on straight-party voting, saying it strikes at the rights of blacks who tend to vote for Democrats with a single mark on the ballot.

U.S. District Judge Gershwin Drain signed an injunction, a week after hearing arguments. He said the law would place a "disproportionate burden" on blacks in the fall election, the first election that would be affected.

Straight-party voting, in which all candidates of a single party are picked with just a single mark, is very popular in Michigan cities with large black populations, especially Flint and Detroit. It's been on the books for more than 100 years and has been a common choice in some counties that are steadfastly loyal to Republicans, such as Ottawa.

But it was repealed in January by Republican Gov. Rick Snyder, who signed a bill approved by the GOP-controlled Legislature.

"The real question that the court must answer is whether the burdens caused by P.A. 268 are in part caused by or linked to social and historical conditions that have produced or currently produced discrimination against African-Americans," the judge wrote. "This question is unavoidably answered in the affirmative.

"African-Americans are much more likely to vote Democrat than other ethnic groups, and many feel this is largely due to racially charged political stances taken by Republicans on the local, state and national level since the post-World War II era," Drain said.

The lawsuit was filed by attorney Mark Brewer, former head of the state Democratic Party, on behalf of three people and a union-affiliated group. He said voters stuck in long lines would be "angry" and "confused" without straight-party voting.

"This judge feels that politics are more important — that's the bottom line," said the law's sponsor, Republican Sen. Marty Knollenberg. "Forty other states have eliminated the check-box option, and it's worked there."

In his 37-page opinion, Drain said it's "irrelevant" what other states have done.

The Michigan attorney general's office said it would file an appeal next week. Assistant Attorney General Erik Grill last week told the judge that no court in the U.S. had stopped prohibitions on straight-party voting.

Sarah Bydalek, president of the Michigan Association of Municipal Clerks, said an appeal "would just be chaos," especially when the November ballot — with or without the straight-party option — must be ready by mid-August. Bydalek, the Walker city clerk, had opposed the new law unless the Legislature also loosened rules on absentee voting to reduce lines. That didn't happen.

When he signed the law, the governor said it was time for voters to "choose people over politics." The law included $5 million for additional voting booths and other polling place improvements, although Drain said the amount was "woefully insufficient."

"There is evidence that it would actually take $30 million, six times the amount appropriated, to adequately combat the long lines," the judge said.

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This story has been corrected to show that there are 40 other states that have banned straight-party voting, not 39.