(Reuters) - Pennsylvania’s top court was set to lay out new congressional voting districts for the state after Democratic Governor Tom Wolf on Tuesday rejected a version drawn by Republican legislative leaders as unfairly skewed in their party’s favor.

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In a 5-2 party-line vote, the Pennsylvania Supreme Court’s Democratic majority last month invalidated the existing map as an unconstitutional gerrymander, ruling that Republican lawmakers marginalized Democratic voters to win more seats in the U.S. House of Representatives.

A new map is expected to boost Democrats’ chances of winning several Pennsylvania seats in November’s midterm elections, when they need 24 nationwide to take control of the House from Republicans. Republicans hold 13 of 18 congressional seats in the closely contested swing state.

Legal battles are playing out in several U.S. states over partisan gerrymandering, the process by which district lines are manipulated to favor one party over another. Pennsylvania has long been seen as one of the worst offenders, with one of its more oddly shaped districts nicknamed “Goofy Kicking Donald Duck.”

“The analysis by my team shows that, like the 2011 map, the map submitted to my office by Republican leaders is still a gerrymander,” Wolf said in a statement. “Their map clearly seeks to benefit one political party, which is the essence of why the court found the current map to be unconstitutional.”

Absent an agreement between Wolf and the Republican-controlled legislature, the court plans to draw new lines itself by Monday, with help from an independent redistricting expert. Both sides can submit proposed maps for consideration by Thursday.

Republican state legislative leaders called Wolf’s pronouncements “absurd” in a letter to the governor.

“This entire exercise, while cloaked in ‘litigation,’ is and has been nothing more than the ultimate partisan gerrymander,” wrote House Speaker Mike Turzai and Senate President Pro Tempore Joe Scarnati.

Republicans have also threatened a federal lawsuit challenging the court’s mapmaking authority. The U.S. Supreme Court last week rejected Republicans’ appeal of the state court’s ruling.

Any new map would likely result in incumbents, candidates and thousands of voters suddenly living in new districts, ahead of May’s primary elections.

Wolf’s office retained Moon Duchin, a Tufts University mathematician, to analyze the Republican proposal. In a statement, Duchin said she calculated there was a 1-in-1,000 chance that a map drafted to comply with the court’s order would result in such a large advantage for Republicans.

“The proposed Joint Submission Plan is extremely, and unnecessarily, partisan,” she said.