Virginia’s top court on Friday struck down a congressional map drawn by Democrats and recently approved by voters, dealing a major blow to the party as it struggles to keep pace with Republicans in the nation’s redistricting battle.

The ruling will wipe out four newly drawn Democratic-leaning U.S. House districts in Virginia and means that Republicans will enter the midterm elections with a structural advantage from their moves to carve out more red districts across the country.

Congressional maps have for generations been drawn once a decade, after the census, to account for population shifts. But last year, President Trump started a rare, mid-decade gerrymandering war when he persuaded Texas officials to draw a new map to help Republicans as they face midterm headwinds. California countered with a map favoring Democrats. Other red and blue states followed.

After the Virginia map passed in a statewide referendum late last month, Democrats thought that they had battled Republicans to a draw, or that they had even eked out a small advantage. Then a ruling by the U.S. Supreme Court prompted several Southern states to work to pass new maps, which will favor Republicans.

Now, the rejection of the new Virginia map means that across the country, Democrats stand to lose half a dozen safe seats, and possibly more, from redistricting alone.

Still, Republicans face a challenging political environment in their bid to retain control of their slim House majority, including worries about the economy, the unpopular war with Iran, high gas prices and Mr. Trump’s sagging approval ratings.

In its 4-to-3 decision, the Virginia Supreme Court wrote that Democratic legislators had violated the state’s constitution with their move to enact a new map meant to give their party 10 out of the state’s 11 U.S. House seats, up from the six it currently controls. Virginia voters approved a constitutional amendment to allow for the map in a referendum.

The problem, the court’s majority suggested, was that the first vote on the amendment in the General Assembly, which would authorize Democrats to redraw the map, occurred days before last fall’s legislative elections — meaning that some Virginians who cast their ballots early did so without knowing how their state lawmakers would vote on the new map.

That, the justices wrote, violated the process in the State Constitution.

“This constitutional violation incurably taints the resulting referendum vote and nullifies its legal efficacy,” the majority wrote.

Mr. Trump and Republicans celebrated the decision.

“Huge win for the Republican Party, and America, in Virginia,” the president posted on his social media site.

Democrats seemed despondent over the decision after eight months and nearly $70 million invested in passing the referendum.

Representative Hakeem Jeffries of New York, the House minority leader, who lobbied Virginia legislators to advance their redistricting push and then campaigned for the referendum, said that “the decision to overturn an entire election is an unprecedented and undemocratic action that cannot stand.”

He added: “We are exploring all options to overturn this shocking decision.”

What those options are was not clear in the immediate aftermath of the decision.

Some legal experts believe that the Virginia Supreme Court’s ruling may be the final word on the state’s maps before the election. That is because the case involved a state law challenge about whether state lawmakers had followed rules laid out in the Virginia Constitution, not a question of federal law or the U.S. Constitution.

Gov. Abigail Spanberger, a Democrat, said in a statement that “I am disappointed by the Supreme Court of Virginia’s ruling, but my focus as governor will be on ensuring that all voters have the information necessary to make their voices heard this November.”

Since the U.S. Supreme Court’s ruling late last month that further weakened the Voting Rights Act, Republicans in Tennessee, Alabama and Louisiana have taken steps to draw new maps before the midterms. Those efforts could net Republicans a handful of additional safe seats before voters cast a ballot in November. South Carolina is also exploring a new map before November.

While Democrats have themselves grown more ruthless about gerrymandering, they are broadly struggling to keep up.

In part that is because years ago, some Democratic-controlled states like Virginia installed independent commissions to oversee their map-drawing process in an effort to insulate it from politics. But Republicans kept the power in state legislatures, allowing states like Texas, Florida, North Carolina and Missouri to enact partisan maps with few logistical hurdles.

In Virginia, voters approved the amendment to override the independent commission by about three percentage points after the General Assembly had passed it twice. But Republicans challenged nearly every aspect of the process. Most of these lawsuits were filed before in a county court in the rural southwestern corner of the state, where a judge repeatedly ruled in the Republicans’ favor. These rulings were appealed to the State Supreme Court.

In lawsuits, Republicans argued that the language in the amendment was misleading, that the new districts were not drawn compactly, that it was improper to vote on redistricting at a legislative session that had convened to discuss budget issues and that a state law required county clerks to post notices about the amendment months before it was actually voted on.

One of the most critical questions concerned the sequence of events in Virginia’s complex amendment process. Before voters weigh in on an amendment to the State Constitution, the General Assembly must approve it twice, with an election for the state’s House of Delegates taking place between the two votes. The first vote for this amendment was on Oct. 31, just days before the state election. With hundreds of thousands of Virginians having already voted, Republicans argued that the legislative action had come too late.

The court sided with that argument.

“Early Virginia voters unknowingly forfeited their constitutionally protected opportunity to vote for or against delegates who favor or disfavor amending the Constitution by not anticipating a legislative vote on a constitutional amendment four days before the last day of voting,” the court’s majority wrote in its ruling.

But Democrats’ loss in Virginia is likely to only further stoke more redistricting battles. Already, the party’s lawmakers in New York and Colorado have signaled a desire to try and redraw their maps before the 2028 elections, and Virginia Democrats are likely to be in a similar position, since the court mainly took issue with the process, not with the resulting map.

Abbie VanSickle contributed reporting.