A second-term Republican congressman announced Monday he would not run for re-election next year, becoming the third targeted GOP member in the past week to retire as the party faces a potentially difficult fight to maintain its majority in 2018.

The decision by Rep. Dave Trott of Michigan (pictured) followed retirement announcements last week by Reps. Charlie Dent of Pennsylvania and Dave Reichert of Washington, three members Democrats planned to try to unseat next year. Though all would have been difficult incumbents to defeat, the districts are likely to be much more competitive for Democrats as open races. Several nonpartisan handicappers immediately switched Trott’s district to a “tossup” after his announcement.

Though the trio of retirements hasn’t invited panic among Republicans, it could be a warning sign of a tough political environment facing GOP lawmakers next year. On average, the party in power tends to lose seats in a president’s first midterm election, and Republicans worry that struggling to accomplish their agenda on Capitol Hill and the tumult of President Trump’s first year leave them vulnerable in 2018.

“It’s clear the political environment is trending away from Republicans; the question is, to what extent?" said GOP consultant Ken Spain, who ran communications for the National Republican Congressional Committee during the 2010 cycle. “The next six months are going to tell the story.”

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Early September before an election year typically brings about retirement announcements, as lawmakers evaluate the political climate and their own ambitions. Several factors can put retirement on incumbents’ radar: swing or highly competitive districts, committee chairmanships at the end of their term limits, frustration with lack of accomplishment, or simply a decision to exit after decades in Washington. Congressional leaders keep tabs on these members and aim to either incentivize them to stay, often with committee assignments or fundraising help, or focus on recruiting a similar kind of politician to run for the seat.

Yet such incentives are increasingly difficult to come by, especially for a party that already controls all levers of the federal government. A perceived lack of legislative victories, combined with the grueling nature of perpetual political campaigns, could make retirement more attractive.

“This is the largest majority the Republicans have had since Herbert Hoover. With the great expectations, there is also lot of frustration over what has been a lot of different views and positions within the Republican House in particular,” said former GOP Rep. Tom Reynolds, who chaired the NRCC during the 2006 cycle, when a Democratic wave swept the House.

While some members may feel demoralized, Republicans caution that it is still early in the cycle, and there are a variety of factors in play on both sides.

“It’s too early for the Republicans to say that the amount of retirements is not a problem, and it’s also too early for Democrats to say these are huge opportunities,” Reynolds said.

Republicans point out that several Democratic districts have also been opened up by retirements: in Minnesota, where Rep. Tim Walz is running for governor, and in Nevada, where Rep. Jacky Rosen is running for Senate. Arizona Rep. Kyrsten Sinema is contemplating a run for Senate, which could open another swing district.

But there are likely more retirements on the horizon for Republicans as well. Seventeen GOP members have announced they’re leaving the House after next year -- 10 who are leaving relatively safe districts to run for higher office, and seven who are retiring, which is well below normal, according to an average cited by Roll Call. Still, the three retirements of the past week -- plus Florida Rep. Ileana Ros-Lehtinen, who announced her decision months ago -- are in swing districts that could be critical to deciding control of the House.

Democrats are increasingly optimistic about their 2018 odds, buoyed by a high volume of recruits in a diverse set of districts. Rep. Ben Ray Lujan, chairman of the Democratic Congressional Campaign Committee, said each retirement just increases the party’s chances.

"Republicans having to defend seats that they’ve never had to defend before,” Lujan said. “That does not bode well for [NRCC Chairman] Steve Stivers. He already had a tough map to defend. Now he has three more very expensive districts to try to defend, when we have an advantage in those districts.”

Stivers, however, dismissed any concern about the newly open seats, arguing that the NRCC was already recruiting strong candidates in each of those districts, and that Democrats leaving certain swing districts would open up seats for Republicans as well.

“Retirements will impact 2018 and there will be more retirements, but there will be more Democrat retirements, too,” Stivers said.

Several reports, including one in the New York Times, have listed Republican Rep. Fred Upton as a member potentially considering retirement, and several GOP sources said they have heard he’s considering leaving the House -- though he is also considering a challenge to Sen. Debbie Stabenow. Tom Wilbur, Upton’s spokesman, pushed back on those reports, saying in a statement that Upton is “happy with his day job and remains very focused on the bipartisan work he’s doing for Michigan.”

“At this point, retirement is not in the cards,” Wilbur said.

The DCCC listed 12 Republicans on its “retirement watch” -- including both Dent, a centrist who represents a district that voted for Trump by seven points, and Reichert, whose district backed Hillary Clinton by three points and also voted for Obama in 2012. Democrats are keeping tabs on a wider array of Republican members representing a range of districts from New Jersey, New York and Pennsylvania to Michigan and Florida, watching campaign schedules and fundraising for signs of slowing momentum.

Meanwhile, no incumbent senator has announced a retirement yet, making 2018 potentially the first cycle ever without one in the upper chamber. That would be critical for both Democrats, who are defending a number of seats in states Trump won, and Republicans, who are seeking to stay on offense and avoid expanding Democrats’ potential gains.

But the streak may not last: CNN reported this week that Tennessee Sen. Bob Corker, who chairs the Foreign Relations Committee, is weighing whether to stick around, and Corker told reporters in the Capitol Monday that he didn’t have a definite time frame to make a decision.

“We began discussing this in January and it’s now September,” Corker said. “I want to be responsible as it relates to our state and make sure that a substantial candidate has time to run if I don’t.”

The party’s efforts on tax legislation in the coming months could affect the number or pace of retirement announcements and the party’s overall showing in the midterm elections.

“Tax reform is incredibly important here,” said longtime GOP pollster David Winston, noting it’s a legislative policy issue for the party that strikes at the heart of voters’ concerns about the economy, jobs and wages. While Republican lawmakers “would prefer to have a president with majority job approval, the electorate at this point just wants to see outcomes and results,” Winston says.

In addition to policy hurdles, Republicans are also considering the ways in which the 2016 election might have affected the midterm electorate.

"We are in the midst of a major tectonic shift within the Republican Party -- and, frankly, across both parties -- that has created a great deal of uncertainty,” said Spain, the GOP consultant.

"I think you're going to see members retire in some districts where maybe they are concerned the president could be a drag on the ticket,” he said, noting traditional suburban areas. “But there are also districts where Republicans might be able to outperform previous campaigns simply because the party is changing and Trump reflects that.”

James Arkin is a congressional reporter for RealClearPolitics. He can be reached at jarkin@realclearpolitics.com. Follow him on Twitter @JamesArkin.