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 G.O.P. Senate will stop both of those things and continue to put conservatives on the bench at a record pace.”
Mr. Ryan’s exit is a destabilizing blow to Republicans’ 2018 plans on nearly every front. The one-time Republican vice-presidential nominee 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, Mr. Ryan has also been a reassuring figure for the business community and a source of perceived stability for restless lawmakers pondering retirement.
And Mr. 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 Mr. 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.
“Paul is relentlessly positive and wanted to run an ideas-oriented campaign,” said former House Speaker Newt Gingrich. “But I guarantee you that would not have worked this fall.”
But any campaign-trail embrace of angry grievance politics — of the sort that Mr. Trump ran on in 2016 — alarms other Republicans who fear it will only exacerbate their difficulties in the suburbs and create long-term difficulties.
“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.”
Mr. 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 Mr. 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 at fund-raisers now that he is 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 Mr. 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,” Mr. Reynolds said. “When you’re a lame duck, it changes those dynamics.”
And with the candidate filing period still open in 19 states, Mr. 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.
As recently as last week, Mr. Ryan gathered his top political donors in Austin, Tex., to lay out the party’s strategy for the election and seek their financial support. While Mr. Ryan was noncommittal there about his plans for seeking re-election, he did not indicate his exit was imminent.
Trying to reassure his startled colleagues on Wednesday morning, Mr. Ryan told Republicans that he would “run through the tape” with them in the 2018 elections.
But it is unlikely Mr. Ryan will be able to perform his core leadership duties with the same force he has wielded up to this point. Some party strategists had already grumbled after the Texas gathering that his unwillingness to commit to running again was offering an excuse to major donors to not provide substantial contributions to House campaign efforts.
Mr. Davis said the party must now swiftly press its agenda, warning that Republicans cannot simply promote the recent tax overhaul in the face of the steady drip of news from the Russia investigation of the special counsel, Robert S. Mueller III.
“They can’t wait for Mueller and be reactive,” he said. “They have gavels, they ought to be out there passing an infrastructure bill, doing something. You can’t just do four corners.”
If there is a silver lining to Mr. Ryan’s departure, it was voiced by one House Republican in a competitive district who — speaking on the condition of anonymity so as not to offend the speaker — said that Democrats were now deprived of a shiny object they delighted in targeting in campaign ads. And other Republican lawmakers said they had long ago assumed that Mr. Ryan would not be around much longer.
Mr. 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 Mr. Ryan.
Mr. Gingrich said the heirs to Mr. Ryan must quickly seize control or else doom the party.
“There will be a period of depression and confusion lasting anywhere from two to six weeks,” he said. “And then McCarthy and Scalise will realize the burden is on them to save the majority.”
“But if they go run a scared, timid and confused campaign, they’re going to lose the House and be lucky to keep the Senate,” he said.
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 on 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 and urged his soon-to-be-former colleagues to brandish a Ryan-like message in the fall.
More junior lawmakers, too, may take Mr. Ryan’s exit as a bracing reminder of the political environment.
Representative Peter T. King of New York, a long-serving Republican, said Mr. 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 Mr. Ryan, had grown to see him as a kind of political security blanket, Mr. King said. There was a reassurance in trusting that Mr. 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,” Mr. King said, warning: “They’ll have to really learn how to run a real race.”