ANNAPOLIS, Md. — A Maryland judge ruled Friday that the state’s new congressional map is unconstitutional, the first Democratic-drawn map to be struck down by a court this redistricting cycle.

So far courts have intervened to block maps they found to be GOP gerrymanders in North Carolina, Ohio, and Pennsylvania, infuriating Republicans and leading conservatives to push for the Supreme Court to limit the power of state courts to overturn maps drawn by state legislatures.

Judge Lynne Battaglia issued the ruling after a trial last week in which Republican lawmakers contended that Maryland’s congressional map approved by the General Assembly in December violates the constitution by drawing districts that favor Democrats, who control the legislature.

“The limitation of the undue extension of power by any branch of government must be exercised to ensure that the will of the people is heard, no matter under which political placard those governing reside. The 2021 Congressional Plan is unconstitutional, and subverts the will of those governed,” Judge Battaglia wrote.

The judge added that she was entering a judgment in favor of the plaintiffs to reject the map and “permanently enjoining its operation, and giving the General Assembly an opportunity to develop a new Congressional Plan that is constitutional.”

Judge Battaglia has a long history in Maryland’s judiciary. She served as a member of the state’s highest court from 2001 to 2016. She served as Maryland’s U.S. attorney from 1993 to 2001. She was also chief of staff to a former Democratic Maryland senator, Barbara Mikulski, from 1991 to 1993.

An appeal by the state is almost certain. A spokeswoman for the state’s attorney general, Raquel Coombs, said the office is reviewing the decision.

In Maryland, where registered Democrats outnumber Republicans 2-1 and Democrats hold a strong majority in both chambers of the legislature, the GOP has long criticized the map as one of the most gerrymandered in the nation.

“Judge Battaglia’s ruling confirms what we have all known for years — Maryland is ground zero for gerrymandering, our districts and political reality reek of it, and there is abundant proof that it is occurring,” said a spokesman for Fair Maps Maryland, Doug Mayer. “Marylanders have been fighting for free and fair elections for decades and for the first time in our state’s shameful history of gerrymandering, we are at the precipice of ending it.”

The ruling comes under the unusual circumstances of Maryland having a Republican governor in a redistricting year. Governor Hogan, who has long sought reforms to the way the state draws political boundaries, created a separate commission to draw maps for the state’s congressional seats and state legislative districts in hopes of taking politicians out of the process of drawing districts.

Mr. Hogan submitted the maps to the General Assembly, but the legislature moved forward with maps approved by a separate panel that included top legislative leadership, including four Democrats and two Republicans.

Mr. Hogan vetoed the map approved by the legislature in December, saying it made “a mockery of our democracy.” After the judge’s ruling, the governor called on lawmakers to approve the map submitted by the commission he supported.

“I call on the General Assembly to immediately pass the independent Citizens Redistricting Commission maps that were written with accountability and transparency,” Mr. Hogan said in a statement. “This is an historic milestone in our fight to clean up the political process in our state, and ensure that the voices of the people we are elected to serve are finally heard.”

If the case comes before the Maryland Court of Appeals, the state’s highest court, all but one of the serving judges have been appointed by Mr. Hogan.

Chief Judge Joseph Getty last week delayed the state’s primary from June 28 to July 19, as courts weigh challenges to the state’s new legislative map as well as the congressional map.

At trial last week, a witness for Maryland Republicans testified that partisan considerations took over when Democrats drew the map. Democrats currently hold a 7-1 advantage over the GOP in the state’s eight House seats. The new map made the district held by a lone Republican, Representative Andy Harris, more competitive for a Democrat to potentially win.

An elections analyst at RealClearPolitics, Sean Trende, testified as a witness for Republicans at last week’s trial that Democrats “are almost guaranteed to have seven districts and have a great shot at winning that eighth district.”

The trial involved two lawsuits. One was brought by a group of Republican state lawmakers backed by Fair Maps Maryland. The other was brought by the national conservative activist group Judicial Watch.