Republican Rep. Ryan Costello will retire rather than face a difficult re-election in a newly redrawn Pennsylvania district.

The congressman's decision, announced Sunday, deals a blow to the GOP as it tries to keep control of the House. Even before Costello's move, the party faced a challenge in holding the seat after a new congressional map set by the state's Supreme Court made his 6th District more Democratic. Hillary Clinton would have won the new district by about 9 percentage points in 2016.

Retirements typically make elections harder for the incumbent party to win. Nonpartisan election analysis sites say Costello's decision favors Democrats: Cook Political Report and Sabato's Crystal Ball are changing their ratings for the seat to "likely" Democratic from "toss-up."

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Democrats aim to win 24 Republican-held House seats in November to take a majority in the chamber. Pennsylvania could prove crucial to those hopes because two other GOP congressmen from the state are retiring. After Pennsylvania's top court threw out a Republican-drawn district map earlier this year, Democrats will try to compete for five or more GOP-held seats in the midterm elections.

Costello, 41, has represented the suburban Philadelphia district since 2015. In explaining his decision on MSNBC on Sunday night, Costello said the political environment made it difficult for him to criticize President Donald Trump's behavior that he opposed without facing backlash within his party. Opposition to Trump and his policies has largely driven a consistent edge for Democrats in surveys that ask whether voters prefer a generic Republican or generic Democrat.

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Costello also cited the new congressional map as a factor in his retirement.

The local Democrats and the left has 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. And so, that coupled with the fact that every single time that I speak out and voice my disagreement with what the president might do or might say, only then has my pro-Trump Republicans not too pleased with me either. And so, the combination of what I've had to deal with just in terms of having a young family, what the state Supreme Court did which I think was obscene, along with the political environment, has me — led me to the decision that was best for my loved ones, those who love me the most, those who I love the most, that at this moment in time running for re-election is not the prudent course of action. And I say that after a lot of deliberation. It's been a very tough decision for me to make, but I think I'm making the right decision.

In a statement, the head of House Republicans' campaign arm called Costello's retirement a "great loss for both his colleagues and constituents." The congressman's "leadership will be sorely missed," said Rep. Steve Stivers, chairman of the National Republican Congressional Committee.

He expressed optimism about the GOP's ability to hold the seat.

"We will work tirelessly to ensure this seat remains in Republican hands," Stivers said.

House Democrats' campaign arm cast the retirement as much more dire for Republicans. Democratic Congressional Campaign Committee spokesman Evan Lukaske said Costello's move "should set off alarm bells for vulnerable House Republicans."

Costello likely would have faced Democrat Chrissy Houlahan, a businesswoman who served in the Air Force Reserve. She is considered a top recruit for Democrats as the minority party tries to flip Republican districts.

Costello filed for re-election last week as he made his decision about whether to retire. It is unclear whether he will pull his name from the May 15 primary election ballot.

If he does, the GOP will have only one candidate running for the nomination: attorney Greg McCauley.