Rep. Ryan Costello (R-Pa.) won’t seek reelection this year, complicating the party’s chances of holding a seat in the Philadelphia suburbs after a court decision struck down a GOP-friendly map.

“It’s the most difficult decision I can recall having to make,” Costello told MSNBC’s Kasie Hunt on Sunday night. “I have an 8-month old. I have a 4-year old. And it’s a very challenging job, serving in Congress with a young family.

Republicans in Costello’s 6th Congressional District had been told this week that he’d abandon his campaign before the May 15 primary. The decision was first reported by City & State Pennsylvania, shocking Republicans who viewed the 41-year old as a rising star.

While two other Republicans had filed to run in the 6th District, the incumbent had more than $1.3 million in his campaign account and had won his previous races with more than 56 percent of the vote. Both Republicans were running to the right of Costello, a business-friendly legislator who had opposed the GOP’s effort to repeal the Affordable Care Act. Democrats, who had persuaded businesswoman Chrissy Houlahan to run in the new district, now expect to face a weaker Republican nominee in November.

“Costello’s exit should set off alarm bells for vulnerable House Republicans,” said Democratic Congressional Campaign Committee spokesman Evan Lukaske on Sunday night. “[They] will also have to explain to middle-class voters why they’ve given repeated handouts to the rich and biggest corporations.”

Costello had been considering the decision since February, when the Pennsylvania Supreme Court drew a new map to replace a gerrymander that Republicans put into place seven years ago. The 6th District, which had cut through three suburban counties in an L-shape, was reshaped to include all of Costello’s Chester County and part of Berks County.

That change turned a district that had narrowly backed Democrat Hillary Clinton for president into one that had given her a 10-point margin of victory over Donald Trump. Before Costello’s decision, the Cook Political Report rated the 6th District a “toss-up,” and the congressman had called on state legislators to impeach the judges who drew the new map. (Pennsylvania elects judges in partisan elections, and the majority that ruled on the map was mostly composed of Democrats.)

“The state Supreme Court, in a matter of a week or so decided to invalidate the map,” Costello said on Sunday night. “The first time in the history of the republic that a state Supreme Court has done that.”

Last week, as the filing period closed and the Supreme Court opted not to hear a Republican case against the new map, Costello filed to run. Democrats waited to see whether he’d reconsider, especially after the special House election victory of Conor Lamb (D) in a conservative Pittsburgh-area district rattled Republicans.

“If [Lamb] wins, you’re probably going to see another half-dozen Republicans say they’re not running again,” former vice president Joe Biden said earlier this month, after he campaigned for Lamb.

Republicans, who had waged several unsuccessful legal battles to stop the new map, now worry that it could cost them at least two seats in the Pennsylvania suburbs — the 6th District vacated by Costello, and the 5th District where scandal-plagued Rep. Patrick Meehan (R) has already announced plans to retire. After Lamb’s victory, Democrats are 23 seats away from taking the majority in the House. Costello is the 24th Republican congressman to retire this year with no immediate plans to seek higher office.