Another House Republican is calling it quits in 2018.

Rep. Ryan Costello, who represents Philadelphia’s surrounding suburbs — a major Democratic target in the 2018 midterms — announced over the weekend that he will not seek reelection in November. Why? President Donald Trump.

“Whether it’s Stormy Daniels, or passing an omnibus spending bill that the president threatens to veto after promising to sign [it], it’s very difficult to move forward in a constructive way today,” Costello told the Daily Local News, a West Chester, Pennsylvania, newspaper.

Unlike Republican leadership, which has been trying to downplay the possibility of a major blue wave in the 2018 midterm election, Costello has long been more upfront about the political realities under Trump’s presidency: Democrats are energized to retake Congress. In Pennsylvania especially, where Democrats’ chances of flipping seats have only improved after the state’s high court overturned a Republican gerrymander, things are changing — Costello’s district moved from a “toss-up” to “likely Democratic.”

“I think everybody is aware of what the environment is because they deal with Indivisible people outside their office every week protesting,” Costello told Vox in February after a bout of Republican retirement announcements, referring to the progressive group. “I know that I do. There has been more intensity.”

Costello’s announcement signals a growing Democratic momentum. He joins a growing list of House Republicans leaving Congress; 22 have announced retirement this Congress, and a host of others are leaving to seek different political office or have already resigned. Costello is among the shorter, but still significant, list of retiring House Republicans in major swing districts, including California Reps. Darrell Issa and Ed Royce.

Democrats have the potential to win big in Pennsylvania in 2018

Pennsylvania is a major Democratic target in 2018. Once one of the most heavily Republican -gerrymandered states in the country, the state’s Supreme Court has mandated a new congressional map for 2018 — one that will change Costello’s district, which was part of a messy configuration of Philadelphia’s suburbs, from a “toss-up,” to a likely pick up for Democrats in 2018, according to Cook’s Political Report.

As Vox’s Andrew Prokop explained, the new congressional map for Pennsylvania was a big win for Democrats, who are aiming to take back control of the House of Representatives:

- It creates two new districts where Democrats are favored that didn’t exist in the previous map (and in one of those, they’re overwhelmingly favored). - It keeps the same number of very closely divided swing districts that existed before (three). - It changes one district that had been overwhelmingly Republican to be one where the GOP is favored but not entirely certain to win (Trump won the new district by about 9 points). - Overall, it reduces by one the number of safe Republican districts (where Trump won by more than 15 points), and by one the number of lean Republican districts (where Trump won by 5 to 15 points).

Taking back control of the House won’t be easy for Democrats — they need to win around 24 seats, but states like Pennsylvania are a big opportunity.

And already, an unexpected Democratic win in a special election in Pennsylvania’s 18th District, where Trump won by nearly 20 points in 2016, is giving Democrats momentum.

“In some respects, my ego says to run,” Costello told his local newspaper. “But when I look at what is the right decision for those who rely on me and the state of our body politic, I am convinced that no matter how bipartisan and open and transparent I am, there is so much anger out there that it doesn’t matter.”

Why so many House Republicans are retiring

The list of congressional Republicans leaving the House keeps growing.

Each departure has come with its own personal story. Some, like California’s Issa and Royce, are from swing districts; others have finished their terms as committee chairs, or are avoiding political fallout from scandal, like Texas Rep. Joe Barton.

But there are some overarching trends worth mentioning: Most of the congressional retirement announcements have come with a feeling of frustration, an admission that perhaps being in the House of Representatives lacks the influence that most associate with the country’s power center. Congressional leadership has increasingly centralized decision-making away from individual lawmakers, and there’s a growing understanding that House Republicans could slip into the minority after this midterm election cycle.

There’s a tacit understanding that if lawmakers were polled for job satisfaction, the results would be pretty bleak, Jason Roberts, a political scientist at the University of North Carolina Chapel Hill who studies Congress, told Vox in February. These retirements signal a notable upheaval in the Republican Party, and the departures only further reinforce the trend by opening up more seats for Democrats to be competitive in.

Democrats are already claiming to have momentum on their side. There’s some clear evidence to that effect: State elections in Virginia and Wisconsin have resulted in shocking upsets for Republicans, and special elections to fill once deeply conservative congressional seats — like in South Carolina and Georgia — have become more difficult contests. Conor Lamb’s win in Pennsylvania this month was yet another warning sign for Republicans.

Republicans have already had a difficult year while in the majority; the president and White House are perpetually caught in scandal, overshadowing the few legislative efforts that weren’t mired in political infighting.

“We’re talking about porn stars and the president rather than about tax policy or what we need to get done by the end of the year or what should have been in the omnibus,” Costello told MSNBC’s Kasie Hunt.

In other words, Republicans know it will only get worse if they are in the minority.