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.