A Maryland judge on Friday threw out the state's new Democratic-backed congressional map, ruling the district lines overwhelmingly favored the party in violation of the state constitution, and ordered lawmakers to draw a new plan.

The decision was a blow to Democrats, who are already facing an uphill battle to hold their razor-thin majority in the U.S. House of Representatives in November's midterm elections.

The map, which passed almost entirely along party lines in Maryland's Democratic-controlled legislature last December, gave Democrats seven safe seats while endangering the state's lone Republican congressmember, Representative Andy Harris, by redrawing his district's borders.

Anne Arundel County Senior Judge Lynne Battaglia agreed with Republican arguments that the map violated the state constitution's equal protection guarantee, among other provisions, by diluting Republican voters' electoral power.

"The 2021 plan is an outlier and a product of extreme partisan gerrymandering," she wrote in a 94-page opinion.

She gave the legislature until Wednesday to draw a new map. The state's high court has already moved the primary election from June 28 to July 19 as part of separate litigation by Republicans challenging the legality of the state's new legislative maps.

A spokesperson for the office of state Attorney General Brian Frosh, a Democrat, said it was reviewing the decision and had not yet determined whether it would appeal.

Under the U.S. Constitution, states must redraw their congressional districts every 10 years based on population changes. In most states, lawmakers oversee redistricting, which offers an opportunity to manipulate maps for partisan advantage, a practice known as gerrymandering.

Friday's decision is the latest in a series of rulings from state courts challenging lawmakers' unfettered authority to draw partisan maps, after the U.S. Supreme Court said in 2019 that federal courts could not adjudicate such disputes.

Unlike in Maryland, Democrats have mostly been the beneficiaries in court cases this cycle. Judges in Ohio, North Carolina and Pennsylvania have blocked Republican efforts to install more favorable congressional maps in those states.