Mr. Ryan acknowledged that the way he was doing things was not the way things are usually done.

“Some of you wonder why I just can’t do the normal politician thing, which is to run and then retire after the election,” Mr. Ryan said. “That is what I’m told is the politically shrewd thing to do. I considered that. But just as my conscience is what got me to take this job in the first place, my conscience could not handle going out that way.” Mr. Ryan said he pledged to serve his constituents honestly, “and for me to ask them to vote to re-elect me, knowing that I wasn’t going to stay, is not being honest.”

Mr. Ryan’s constituents, however, are not the only ones potentially being spared. His choice to leave the House voluntarily at 48 gives him a chance to limit his culpability for a potential loss of the majority or face its consequences, possibly leaving him viable for a future political run.

“It gives him a chance to spend time with his kids, make some money, and then he will be in his mid-50s and can run for president, if that’s what he wants to do,” said Representative Peter T. King, Republican of New York.

Whether that is what he wants to do is unknown at the moment. During his remarks to the news media, the speaker said he intended to keep pursuing policy ideas that he favored, such as reshaping big social programs.

In listing the “two greatest honors” of his life, he mentioned being elected as speaker and serving as a husband and father. His run for vice president on the national Republican ticket in 2012 never came up. Instead, there was talk around Washington of Mr. Ryan leading some kind of major Washington advocacy group or think tank. He later told CNN that he had no intention of running for the White House.

Should he enter national politics again, Mr. Ryan’s legacy in the House will no doubt be an issue. He was a champion of fiscal responsibility who will be leaving behind huge deficits piling up under full Republican control of Congress and the White House. He pushed through a long-sought tax overhaul that he celebrated again Wednesday. But one of his chief goals — simplification of the tax code by reducing an array of deductions — was watered down in the final negotiations as Republicans hunted for votes for passage.