WASHINGTON — After a stunning apparent special election defeat in the state's southwest, Pennsylvania Republicans face another headache: U.S. Rep. Ryan Costello's potential retirement.

Costello, of Chester County, has warned GOP leaders that he may not run for another term in the House, according to four sources familiar with the conversations, leaving party officials worried about fielding a candidate in a highly competitive district that could flip to Democrats.

The concerns about Costello's future have swirled for weeks, dating to when the Pennsylvania Supreme Court redrew the state's congressional map and made his district, the Sixth, more favorable to Democrats.

Costello is collecting signatures for a potential campaign, but is closely watching the GOP legal challenges attempting to block the new map and restore his old district lines. He raised his doubts about his political future before Tuesday's election, but the apparent Democratic victory that night in a deep-red district outside Pittsburgh added another warning sign for Republican incumbents.

If he joins other Republicans in retiring, Democrats would likely become the favorites to pick up his seat. The race is already seen as a top-tier contest, even with Costello involved, and Democrat Chrissy Houlahan is one of her party's top recruits.

Costello is less than a week away from a Tuesday deadline to file petitions to get on the primary ballot and has not made a definitive statement about his plans. If Costello were to decide not to run now, the GOP would have little time to find a replacement candidate.

"He has not said," Val DiGiorgio, the chairman of the state Republican Party and the Chester County GOP, said Wednesday. "All I know is he's out there, he's raising money, and he's collecting petitions. I'm proceeding under the perception that he's ready to fight for his district."

On Thursday morning, the Chester County GOP sent out an email promoting two events to gather signatures for Costello's reelection bid.

But Costello has answered indirectly when asked if he will seek reelection.

"I said I was circulating petitions," he wrote to the Inquirer and Daily News on Wednesday.

He has pointed to planned fund-raising events, but in response to questions about his campaign has mostly focused on attacking Democrats on the state Supreme Court, calling their new congressional map "racist" and arguing for impeaching and disbarring some of the justices.

The Democratic-controlled court ruled that the old Pennsylvania map was drawn to unconstitutionally favor Republicans.

Republican leaders sounded out at least one alternative candidate in case Costello quits, but that person turned them down, two Republican sources said. They, like others interviewed for this story, requested anonymity to discuss private conversations.

Republicans are already facing stiff headwinds heading into the fall midterm elections, with Democrats targeting several districts in the Philadelphia area. Reps. Pat Meehan of Delaware County, Frank LoBiondo of South Jersey, and Charlie Dent of Allentown, are all retiring, opening opportunities for Democrats.