Four members of the House have said they won't run for reelection and two senators are now weighing their options. (Sarah Parnass,Jenny Starrs/The Washington Post)

A small wave of Republican lawmakers have announced that they will not seek reelection next year, and more are actively considering it, threatening to make what many within the GOP already viewed as a difficult election cycle even harder.

In the House, the early spate of GOP retirements means the party will not enjoy the advantage of incumbency in several closely divided districts — and raises the possibility that many more lawmakers will choose to retire rather than face tough reelection campaigns.

No Republican senators have announced retirement plans yet, but several are considering it amid threats by outside groups backing President Trump to challenge establishment-wing senators in next year’s GOP primaries.

The trend reflects an increasingly competitive landscape next year, fueled in part by a highly motivated Democratic Party eager to reclaim at least one lever of power in Washington. It also comes at a time of legislative gridlock and an increasingly contentious relationship with Trump. Despite the party’s control of government, it just isn’t that fun to be a Republican right now.

Sen. Bob Corker (R-Tenn.) speaks to reporters on Capitol Hill last week. Corker said Monday he was considering not running for a third term next year. (Joshua Roberts/Reuters)

On Monday, Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, publicly wavered on whether he would seek a third full term, while two-term Rep. Dave Trott (R-Mich.) said he would not seek reelection in a suburban Detroit district.

Trott became the third Republican in five days to announce his retirement, joining Reps. Charlie Dent (R-Pa.) and Dave Reichert (R-Wash.). The three all represent swing districts vulnerable to Democratic takeover. Key political forecasters have moved their ratings for each of these seats toward Democrats.

GOP strategists said Monday that the current pace of retirements is in line with historical averages, and that it is too early to tell whether there will be an unusually large number of open seats that will be contested next year. But many are privately wondering if these are the first of a wave of retirements, similar to what was seen in 2006, when Democrats took the House by winning 30 seats, and 2010, when Republicans reclaimed the majority with a 64-seat swing.

“Obviously, there is a mood — with Trump in Washington, with a disorganized caucus that can’t put it together — that’s giving members a reason to pause,” said Thomas M. Davis III, a former Virginia congressman and former chairman of the National Republican Congressional Committee. “The next thing to look at is: What’s Bob Corker do? What’s [Sen.] Susan Collins do? Is she going to run for governor? I think we have to wait and see.”

Democrats see a more attainable path to a majority in the House — where they would have to flip 24 seats out of 435 — than in the Senate, where the GOP’s slim 52-48 advantage obscures a difficult electoral map for Democrats. Even in the Senate, however, infighting and messy primaries could sap GOP resources that might otherwise be aimed at expanding the party’s majority.

Two Senate incumbents, Jeff Flake (R-Ariz.) and Dean Heller (R-Nev.), are facing primary challenges from opponents who are accusing them of being insufficiently pro-Trump. Those races stand to become battles in a war waged by former White House strategist Stephen K. Bannon and conservative financier Robert Mercer against Senate Majority Leader Mitch McConnell (R-Ky.) and the GOP establishment.

Corker, who was a strong supporter of Trump’s campaign but has since delivered measured criticism of the administration’s missteps, has also been dogged by rumors of a primary challenge. He recently said that Trump had not demonstrated the “stability” or “competence” necessary to effectively lead the nation, prompting Trump to return fire on Twitter: “Strange statement by Bob Corker considering that he is constantly asking me whether or not he should run again in ’18. Tennessee not happy!”

After President Trump's most recent rhetoric about Charlottesville inflamed even more criticism, many Republicans stayed silent. But a handful of GOP lawmakers and now Trump's own economic adviser are directly criticizing him. (Jenny Starrs/The Washington Post)

Corker told reporters Monday that his decision would be rooted in personal rather than political concerns, and he sought to downplay Bannon’s potential role in opposing him: “I have no reason whatsoever to believe the administration would encourage a primary.”

Josh Holmes, a former top aide to McConnell, said Monday that Trump’s willingness to attack his fellow Republicans could erode some candidates’ willingness to put themselves through the campaign wringer.

“The takeaway that you have, I think, with some of them is: ‘You know, this may not be a place for me any longer,’ ” he said. “ ‘If the whole point of going to Washington is being famous and saying outlandish things so I can show up on Fox News or do an hour-long ‘60 Minutes’ special, then that’s not why I got into it.’ ”

Individual House members have not been subject to personal attacks from Trump the way Corker, Flake and Heller have been. But pro-Trump insurgents have threatened primary campaigns against several GOP moderates in North Carolina, Pennsylvania and Virginia.

One of those facing a potentially nasty primary was Dent, a frequent critic of both Trump and the party’s hard-right faction. Dent cited the political landscape in explaining his decision to retire last week.

“Accomplishing the most basic fundamental tasks of governance is becoming far too difficult,” said Dent, whose potential primary challenger had gotten a boost from Bannon’s Breitbart News. “It shouldn’t be, but that’s reality.”

Neither Reichert nor Trott cited broader political concerns in announcing their decisions to step away from Congress; neither did Rep. Ileana Ros-Lehtinen (R-Fla.), another House moderate who said in April she would retire.

But each seat is now a potential Democratic pickup opportunity in a way it was not a week ago, said David Wasserman, who analyzes House races for the nonpartisan Cook Political Report. Wasserman said it is too early to draw firm conclusions about the 2018 midterms, but he noted that an early spate of majority-party retirements was a bellwether for the 2006 and 2010 waves.

“It provided the early momentum in both cases, and it reshaped the narrative about whether the House was in play early on, and I think that’s what it’s doing right now,” he said.

The Democratic Congressional Campaign Committee has indeed touted the retirements as evidence of a map tilting steadily in Democrats’ direction. The group has long targeted Reichert and Ros-Lehtinen, in particular, whose districts tend to support Democratic presidential candidates but whose personal popularity has produced comfortable reelection margins.

Open seats will change that dynamic, though Davis warned that Republicans were able to retain a hotly contested open seat in a Georgia special election earlier this year. “This is increased opportunity for Democrats, but that’s all it is,” he said. “This is no slam dunk by any means.”

House watchers are now eyeing veteran lawmakers in a number of closely divided districts, many of them in states that lean Democratic. They include Rep. Rodney Frelinghuysen (R-N.J.), the Appropriations Committee chairman, and Rep. Edward R. Royce (R-Calif.), the Foreign Affairs Committee chairman, as well as Reps. Darrell Issa (R-Calif.), Dana Rohrabacher (R-Calif.) and Frank A. LoBiondo (R-N.J.).

A GOP official said several of those rumored retirees have ratcheted up their fundraising and show no signs of stepping aside. But the retirements already announced are a fresh warning sign to GOP congressional leaders that they must redouble their efforts to deliver on their legislative agenda after attempts to repeal the Affordable Care Act ended in an embarrassing defeat. Trump has increased pressure this month on McConnell and House Speaker Paul D. Ryan (R-Wis.) to deliver a tax overhaul in the coming months.

Party strategists see a tax cut as a key element in any effort to keep GOP control of Congress — and worry that failure to make some headway could encourage more lawmakers to inch toward the exits.

“If we don’t see some substantial progress on issues like tax reform by the end of the year, I think a large number of members may start reconsidering their options,” said Michael Steel, a political consultant who served as a top adviser to then-House Speaker John A. Boehner (R-Ohio).

For now, the GOP’s effort to hold the line on retirements is probably happening on a quiet, member-to-member basis. Thomas M. Reynolds, a former New York congressman who led the NRCC during the 2006 election cycle, said that retirement decisions tend to be intensely personal, but party leaders can try to stave them off.

“You carry in your breast pocket [a list of] ‘suspects’ that might think about retiring, and you have discussions with ’em,” he said. “Some you’ll be able to help, and some you won’t.”

McConnell has learned about the limitations of a personal appeal in maximizing his party’s electoral chances. In talks with Trump, McConnell has tried to emphasize the importance of expanding the GOP Senate majority to deliver on the party’s agenda, according to two Republicans familiar with the conversations who spoke on the condition of anonymity to describe private conversations.

He suggested months ago that Trump consider appointing Sen. Joe Manchin III (D-W.Va.) or Heidi Heitkamp (D-N.D.) to his Cabinet, one of the Republicans said. Trump did not take the advice. Instead, last week the president invited Heitkamp to fly with him on Air Force One — lending her some bipartisan credibility in a state where she will need to win conservative voters.

Sen. John Cornyn (Tex.), the second-ranking Republican and a former Senate GOP campaign chief, said Monday that he wished Trump, Bannon and their allies would refrain from attacks against GOP lawmakers and focus more on attacking Democrats.

“The president’s going to need as many friendly faces around here as he can get in order to get things done,” Cornyn said, echoing an argument one of the Republicans said McConnell has made to the president in private. “And I realize bipartisanship is important. But you shouldn’t mistake a smile for support when it really counts.”