WASHINGTON — One after another, critical congressional races in the Philadelphia region have fizzled before a single vote has been cast.

A string of Republicans have turned away from tough contests, leaving Democrats with opportunities to net a chunk of the House seats they hope to gain this fall.

The latest example came Sunday night, when Rep. Ryan Costello, a Chester County Republican, decided against running for reelection, bowing out of a contest once heralded as a national bellwether for control of the House.

Costello's departure had been brewing for weeks, as he grew angry over a new congressional map imposed by the Pennsylvania Supreme Court and, according to confidants, had soured on serving in Congress.

To keep the seat, he would have had to overcome a strong challenger in a newly redrawn district and an area reflecting what has been a suburban backlash against President Trump.

"The local Democrats and the left have become more engaged and candidly more angry by the week as President Trump says things and does things which many Republicans, myself amongst them, from time to time do disagree with," Costello told MSNBC Sunday night in announcing that he would not seek another term representing the state's Sixth District.

While Trump remains popular in parts of Pennsylvania, Republicans say their internal polls show deep disapproval in the suburbs — and the same actions that thrill his base have stirred intense Democratic turnout in elections.

Costello, 41, also told MSNBC the job was straining his family life.

"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," he said, lamenting that even routine work has become onerous.

Several people close to him said the congressman was discouraged by protests he encountered, blasts on social media, and memories of last year's attack by a gunman at a Republican baseball practice in Virginia. Costello was supposed to be on the field that morning.

"He has said a number of times, 'But for missing my car, I would have been out there,'" said Alan Novak, a former Republican state chairman from Chester County.

Costello's departure, days after the deadline for candidates to get on the ballot in Pennsylvania, left local Republicans groping for a way forward and frustrated that he decided so late. Chester County GOP leaders huddled Monday to discuss next steps, including whether they can get another name on the ballot.

The deadline for Costello to ask to be removed from the ballot is Tuesday. Party officials did not announce any plans but recruiting a replacement this late could prove daunting.

"Given the timing of his announcement and my firm belief that he could win re-election, I am disappointed by his decision not to seek re-election," Chester County and Pennsylvania GOP Chairman Val DiGiorgio wrote to party supporters.

Republicans saw Costello as their best chance of keeping the seat in the face of midterm headwinds and a strong Democratic challenger, Chrissy Houlahan.

Without the advantage of incumbency, election analysts give Republicans little chance of holding the seat. Under the recent redistricting, Costello's district went from one that Democrat Hillary Clinton won by less than a percentage point to one she would have won by nearly 10.

"There are a lot of folks that are disappointed," said Jim Gerlach, a Republican and Costello's predecessor in the seat. "The timing of it is not good relative to having a good chance to hold the seat in a very competitive year."

Costello's departure follows the retirement announcements of Rep. Pat Meehan of Delaware County, Rep. Frank LoBiondo of South Jersey, and Rep. Charlie Dent of Allentown.

All faced tough reelection challenges, but also represented Republicans' best hopes of holding swing seats.

"Some of the key building blocks for a potential Democratic House majority are in the Philly media market," said Kyle Kondik, a congressional election analyst at Sabato's Crystal Ball.

The site rates Democrats as the favorites in six seats held by the GOP nationwide — three based in the Philadelphia suburbs: in Chester County, Delaware County and LoBiondo's sprawling South Jersey district.

The Cook Political Report rates four Philadelphia-area seats among the eight most vulnerable ones held by Republicans.

Reps. Brian Fitzpatrick of Bucks County and Tom MacArthur of South Jersey are now the region's only Republicans seeking reelection in an area where the GOP has dominated suburban races since 2010. Democrats now have an easier road to victory in the Philadelphia area, freeing up resources that they can use against Fitzpatrick or MacArthur.

Fitzpatrick's race is widely seen as a toss-up.

The GOP is going to lose seats, Novak said. "It's a question of how many." .

In all, more than 40 House Republicans are not seeking reelection, are running for another office or have already resigned this election cycle.

Costello's race had also been seen as a toss-up before he dropped out — and the head of the Republican congressional campaign arm said last year that his race was one to watch to gauge who might win control of the House.

That, however, was before the Pennsylvania Supreme Court imposed new district boundaries across the state. Costello veered from boasting that he could still defeat Houlahan to railing against the state judges as "corrupt" and hoping a federal court would block their decision.

That hope was dashed by the courts last week, a day before petitions were due, leaving Costello to file for a race his heart wasn't in.

Another Republican, attorney Greg McCauley, has also filed to run and could become the party's nominee.