WASHINGTON — Fifteen months after Republicans took full control of Washington, the man long seen as central to the party’s future is abandoning one of the most powerful jobs in the capital, imperiling the GOP grip on the House and signaling that the political convulsions of the Trump era are taking a grave toll on the right months before Election Day.

House Speaker Paul Ryan announced Wednesday that he will not seek reelection in November. Ryan, 48, said he will serve until the end of this Congress in January, which will mark 20 years in Congress. He insisted he will be “leaving this majority in good hands with what I believe is a very bright future.’’

But his announcement blindsided many House Republican candidates and their campaign leaders who were counting on him to lead them to victory in the November midterm elections. His decision to leave Congress sent an undeniably pessimistic message to Republicans: that stable, steady leadership is lacking in their deeply divided party as they head into a campaign season defined by the whims of President Trump.

Trump offered well wishes on Twitter before a planned dinner with Republican congressional leaders at the White House on Wednesday evening.

Representative Nancy Pelosi of California, the House Democratic leader who longs to return to the speakership, was faint with her praise. “The Speaker has been an avid advocate for his point of view and for the people of his district,’’ she said in a statement. “Despite our differences, I commend his steadfast commitment to our country.’’

For a White House bracing for a potential Democratic impeachment inquiry, the ominous impact of Ryan’s retirement was unmistakable: He has made it more difficult to stave off Democrats’ taking control of the House, where Republicans currently hold a 23-seat majority.

As many as 50 House Republican seats are at risk in competitive races this year. Private polling indicates that Trump’s approval rating is well below 40 percent in some of those tossup districts, the sort of low political standing that often dooms candidates of the president’s party.

“This is the nightmare scenario,’’ said former representative Thomas M. Davis, Republican of Virginia. “Everybody figured he’d just hang in there till after the election.’’

Already, some veteran Republicans are suggesting that the party shift its focus from the House to protecting its one-seat Senate majority.

“It seems clear now that the fight is to hold the Senate,’’ said Billy Piper, a lobbyist and former chief of staff to Senator Mitch McConnell, the Republican leader. “The first thing a Democrat House majority would do is begin impeachment proceedings. The second would be to undo tax reform. A GOP Senate will stop both of those things and continue to put conservatives on the bench at a record pace.’’

Ryan’s exit is a destabilizing blow to Republicans’ 2018 plans on nearly every front. A onetime Republican vice-presidential nominee, he has been the party’s most important fund-raiser in the House, attending fund-raisers nearly every night he is in Washington and raising more than $54 million so far for this election. In contrast to a president who embraces chaos, Ryan has also been a reassuring figure for the business community and a source of perceived stability for restless lawmakers pondering retirement.

And Ryan has been the most important voice on the right calling for an upbeat and inclusive message and a campaign focused on the economy and taxes, rather than the hard-right culture war issues Trump delights in stoking.

Now, some in the party are suggesting that the speaker’s departure will free Republicans to run a more hard-edge campaign that better reflects the politics of the man in the Oval Office.

But any campaign-trail embrace of angry grievance politics — of the sort that Trump ran on in 2016 — alarms other Republicans who fear it will only exacerbate their difficulties in the suburbs and create long-term problems.

“This is a huge moment of truth,’’ said Representative Tom Rooney of Florida. “I don’t think that campaigning or governing by fear is ever going to work or ever going to be a lasting message. You can only scare people so much. And if we try that, we’re not going to be in power much longer.’’

Ryan indicated to advisers that he knows retiring will create political difficulties for the party but that he felt he could not in good conscience commit to another full two-year term, according to two Republicans familiar with the conversations.

Yet his explanation that he wanted to spend more time with his three teenage children, as expressed at a news conference Wednesday, is of little comfort to Republicans on the ballot who were expecting Ryan to raise millions for and campaign with lawmakers across the country. Even though he vowed to colleagues that he would keep fulfilling those political responsibilities, he will not be nearly as big a draw as a lame duck.

Former representative Thomas M. Reynolds of New York, who sits on the board of a Republican outside spending group tied to the speaker, said that Ryan had effectively scrambled the party’s fund-raising machinery.

“It will be a difficult task for Paul to hold his strong, vibrant fund-raising,’’ Reynolds said. “When you’re a lame duck, it changes those dynamics.’’

And with the candidate filing period still open in 19 states, Ryan has lost any real power to convince other wavering Republicans that they must run again.

More than three dozen other Republicans are leaving the House to retire or seek other offices, and several more have resigned in personal scandals or for private-sector jobs.

Ryan’s announced exit also threatens to divide the rest of the Republican leadership team in the House: the second- and-third-ranking House Republicans, Kevin McCarthy of California and Steve Scalise of Louisiana, are competing to succeed Ryan.

In a sign that Republican retirements are likely to continue, Representative Dennis A. Ross of Florida, who holds a conservative-leaning but not safe seat, announced Wednesday morning that he would leave at the end of his current term. He said on CNN that the negative atmosphere in Washington was “a factor’’ in his decision.

More junior lawmakers, too, may take Ryan’s exit as a bracing reminder of the political environment.

Representative Peter T. King of New York, a long-serving Republican, said Ryan had played down the impact of his decision and predicted that no one would “win or lose an election based on whether Paul Ryan is the speaker.’’

But newer members, who may never have served under a speaker other than Ryan, had grown to see him as a kind of political security blanket, King said. There was a reassurance in trusting that Ryan “would be there if they needed campaign contributions,’’ he added.

“It was just a comfort zone, knowing that Paul Ryan was there, for a lot of these people,’’ King said, warning: “They’ll have to really learn how to run a real race.’’