HARRISBURG — A political scientist serving as an expert for voters challenging Pennsylvania’s congressional districts testified Monday that “extreme partisan intent” by Republicans appears to have been the predominant factor in producing a map that has disproportionately favored GOP candidates.

University of Michigan professor Jowei Chen said during the first day of a Commonwealth Court hearing over the 2011 maps that none of the hundreds of computer simulations he has run has produced a map so favorable to Republicans.

In recent elections, Republicans have had a durable 13-5 advantage among the congressional delegation, and the lawsuit claims the maps are so partisan they violate the state constitution.

“Whichever way you slice and dice the data, the enacted plan is a 13-5 Republican plan,” Chen said.

Commonwealth Judge Kevin Brobson is presiding over the trial that began Monday and could last all week. But Brobson will not decide the case. Instead, he said, he will produce proposed facts and legal conclusions to the state Supreme Court to use in resolving the case.

The lawsuit by a group of voters alleges violations of the state constitution’s free expression and association clauses by discriminating against the plaintiffs and other Democratic voters. They are seeking new maps for next year’s races.

Pennsylvania has about a million more registered Democrats than Republicans, and Democrats hold the governorship and all three statewide row offices. The state has one Democrat and one Republican in the U.S. Senate, and statewide judicial races have gone for both parties in recent years. Republicans, who have controlled redistricting and reapportionment for several cycles, hold overwhelming majorities in both chambers of the Legislature.

A separate lawsuit challenging Pennsylvania’s congressional district maps went to trial last week in federal court and awaits a decision by a three-judge panel. The U.S. Supreme Court has also heard oral argument in a closely watched challenge to Wisconsin’s state legislative districts and is expected to establish national standards for gerrymandering cases.

In court Monday, Chen described how he generated 500 maps of the state’s 18 congressional districts based on traditional principles of equal size, compactness and minimal municipal or county splits. The vast majority of those simulations produced eight or nine Republican districts.

By contrast, actual voting patterns from elections for those seats in 2012, 2014 and 2015 have produced 13-5 splits in every case, Chen noted.

Lawyers for the Republican leaders who are among the defendants, House Speaker Mike Turzai and Senate President Pro Tempore Joe Scarnati, said they plan to put on a rebuttal expert to challenge parts of Chen’s testimony.

Brobson, an elected Republican, said he sees his role as serving as a hearing officer and emphasized that the state Supreme Court will decide the case, not him.

The lawsuit, filed by Democratic voters from each district, described the maps as “a districting plan that is utterly unresponsive to — and often flouts — the will of voters.”

Part of their claim involves “tortured shapes” they see as evidence of “raw partisanship.”

The state’s 7th congressional district, for example, they said, “has been dubbed ‘Goofy kicking Donald Duck’ to reflect its absurd shape, and in some places is so narrow that the only thing holding the district together is a steakhouse … or a medical endoscopy center.”

Their case had been put on hold until Nov. 9, when the justices, by a 4-3 vote, put it on a fast track . They gave Brobson until Dec. 31 to conclude the proceedings. The high court’s two Republicans, justices Thomas Saylor and Sallie Mundy, as well as Democratic Justice Max Baer, dissented from the order.