On Thursday night, Dan Helmer received a shipment of boxes with 1,000 yard signs that read: “Dan Helmer for Congress.”

By late Friday morning, Mr. Helmer no longer had a seat to run for.

The whiplash for the Virginia Democrats running for Congress was swift and intense after the state Supreme Court struck down the new congressional map proposed in February to flip four Republican-held seats.

With the stroke of a pen in Richmond, some campaigns effectively went poof, other candidates suddenly were in far tougher districts and one went from on the verge of dropping out to gearing up for a long-shot battle in a deep-red part of the state.

Rarely have so many fully formed campaigns gone off the rails at once. The court’s shock decision on Friday dashed Democratic hopes of providing some balance to Republican-run states that have been eliminating Democratic seats since Texas kicked off a nationwide fight last year.

Mr. Helmer, a senior member of the House of Delegates, was an architect of Virginia’s redistricting gambit that began in October. His colleagues subsequently split up Northern Virginia and created a new lobster-shaped Democratic seat ideally suited for him. Barring a miracle from the Supreme Court, Mr. Helmer said his congressional campaign is most likely over.

“There’s no seat for me,” he said. His new yard signs “are probably not as useful as they were yesterday.”

Tom Perriello, a former congressman who later served as a diplomat in Africa during Barack Obama’s presidency, began his campaign in December with the expectation that new maps were coming.

He woke up Friday morning in his home near Charlottesville in a district that Vice President Kamala Harris carried by three percentage points in 2024. Once the court ruling came a few hours later, he lived in a district Mr. Trump won by 12 points.

Mr. Perriello said he would now run against Representative John McGuire, a first-term Republican whose district covers conservative Southside Virginia. He had planned to run in a district that stitched together small Democratic-leaning cities and college towns in the Shenandoah and Blue Ridge Mountains.

The situation is even more jarring for some Democratic voters, Mr. Perriello said.

“I just walked into a food pantry in the Shenandoah Valley and the African-American woman who runs it broke down in tears and said for the first time in her life she thought she was going to have representation,” Mr. Perriello recalled Friday. “This is what the last two months have been about, about hope for the first time for people.”

Some hope Democrats will gain seats even without the new maps.

Virginia Democrats now hold six of the state’s 11 House districts. President Trump won two of the other five by five points or less, making the Republican incumbents who represent them, Representatives Jen Kiggans and Rob Wittman, endangered given the headwinds the G.O.P. faces. Their Democratic opponents, former Representative Elaine Luria and Shannon Taylor, a local prosecutor, entered the race well before the redistricting push and remain top-tier challengers.

Representative Hakeem Jeffries of New York, the House minority leader, said in an interview Friday that “we’re going to pick up at least two seats” in Virginia under the existing maps.

Jeff Ryer, the chairman of the Republican Party of Virginia, said he had given the state Supreme Court case a 50-50 chance of succeeding. Democrats, he argued, have been overconfident in the state ever since Abigail Spanberger won last year’s governor’s election by more than 15 points.

Now, instead of a map designed to hand 10 of 11 seats to Democrats, Mr. Ryer said he expected multiple competitive campaigns this year. He allowed that it would not be easy defending the seats Mr. Trump narrowly won.

“These districts are designed to be compact and contiguous, and when you draw them that way you’re going to get some that are not competitive and some that are hotly contested,” Mr. Ryer said.

Some Democrats vowed to soldier on under difficult conditions.

Beth Macy, the best-selling author of books like “Dopesick,” about the struggles in Appalachia, started her campaign in November before the proposed Democratic maps drew her into a district with Mr. Perriello, who was far better known and had deeper connections in their shared region of Virginia.

Ms. Macy said on Friday that she had been considering conceding the primary and endorsing Mr. Perriello, but now would remain a candidate against Representative Ben Cline, a Republican whose district includes her hometown, Roanoke.

“I feel bad, but you know, we can’t just roll over,” Ms. Macy said. “Democrats have got to stop showing up to a knife fight with a spork.”

Ms. Macy, who is now running in a deep-red district Mr. Trump carried by 25 points, is not short on optimism. She predicted the backlash to Republican policies in Washington could lead to Virginia Democrats sweeping out G.O.P. incumbents across the state and perhaps reach the same end Democrats had hoped for with their maps.

“We have never been to where we are in this country,” she said. “It’s a national emergency.”