Patrick Marley | Milwaukee Journal Sentinel

MADISON - Assembly Speaker Robin Vos must testify in Wisconsin's gerrymandering case, a divided federal court ruled Friday.

The Republican leader of the lower house plans to appeal the ruling, said his spokeswoman, Kit Beyer.

Vos spent weeks fighting the effort to force him to answer questions under oath about legislative maps lawmakers drew in 2011 to help the GOP win elections. But in a 2-1 decision Friday, the judges concluded Vos had to give a deposition and turn over documents.

"We acknowledge that a sitting legislator is not subject to civil process in any but the most exceptional circumstances. But this is an exceptional case that raises important federal questions about the constitutionality of Wisconsin’s plan for electing members of the Assembly," the majority wrote.

"Vos was a key figure in enacting that plan and he was involved at nearly every stage of the process. Probably no one has a better understanding of the challenged (redistricting) plan than he does."

Republicans and Democrats have been fighting over the election maps for eight years, ever since Republicans used their complete control of state government to establish legislative boundaries that helped them keep their majorities. There have been two lawsuits over the maps and one trip to the U.S. Supreme Court.

In the latest feud, Democratic voters who sued over the maps sought to depose Vos, of Rochester. He argued he didn't have to answer questions or turn over documents because lawmakers are generally immune from civil litigation under the state constitution.

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The court's majority rejected that argument because of the seriousness of the legal fight and because information Vos has about lawmakers' intentions "is not only relevant but also necessary" to the case.

Tuesday's decision was issued by Judges Kenneth Ripple and James Peterson.

In dissent, Judge William Griesbach contended Vos should have been granted immunity because the case is about partisan gerrymandering — "a practice that is older than the Republic" — rather than acts that courts have found to be illegal.

Redistricting cases are unusual because they are initially heard by a special judicial panel instead of a single judge.

Ripple sits on the 7th Circuit Court of Appeals in Chicago and was appointed by President Ronald Reagan. Peterson is a district judge based in Madison and was appointed by President Barack Obama.

Griesbach is a district judge in Green Bay and was appointed by President George W. Bush.

A trial is scheduled for July, but before then the Supreme Court is expected to issue decisions in gerrymandering cases from Maryland and North Carolina. Those rulings could have major implications for Wisconsin's case — either by improving the Democrats' chances or by rendering their arguments moot.

In the first lawsuit over Wisconsin's maps, a panel of three federal judges in 2012 found the district lines violated the voting rights of Latinos on Milwaukee's south side and ordered changes to two maps.

In the other case, a different set of judges in 2016 ruled that Wisconsin’s maps violated Democrats’ voting rights because they were so heavily tilted to Republicans’ favor.

The Supreme Court last year found that those who brought that case hadn't proved they had legal standing to sue and sent the case back to the lower court. Friday's ruling came in that case as all sides prepare for trial.

States must draw new congressional and legislative districts every 10 years to account for population changes. Regardless of how the courts ultimately rule, Wisconsin officials will have to draw new maps in 2021, after the next census.