The League of Women Voters of Pennsylvania filed a lawsuit Thursday alleging that the Keystone State’s congressional map was illegally drawn.

“This case is about one of the greatest threats to American democracy today: partisan gerrymandering,” the plaintiffs, which include the group and several voters, wrote in their complaint.

The plaintiffs allege that the Pennsylvania legislature’s Republican majority drew the congressional map to benefit the GOP by packing Democrats into five congressional districts, and spreading them out among the remaining districts. Republicans currently hold 13 of the state’s 18 U.S. House seats.

In the complaint filed in the state’s Commonwealth Court, the plaintiffs cite a Brennan Center for Justice report that said the Pennsylvania map was one of the “extreme” cases of partisan gerrymandering.

As an example, they point to 2012 when Republicans won 49 percent of the statewide congressional vote, but took 72 percent of the congressional seats (13 out of 18).

Pennsylvania is home to a handful of Democratic Congressional Campaign Committee targets in 2018. The National Republican Congressional Committee is also targeting Democratic Rep. Matt Cartwright.

A few other states are in the midst of redistricting conflicts ahead of next year’s midterms. A case in Texas, moving forward this summer, centers on racial gerrymandering claims. The Supreme Court recently rejected two of North Carolina’s congressional districts over the illegal use of race in redistricting. Maryland has an ongoing lawsuit as well.