By Friday afternoon, party leaders on both sides were already weighing new names — among them more widely known possibilities, including a state senator and a former candidate for the seat. Candidates have until May 11 to submit petitions to run in an Aug. 3 primary.

“Obviously we would have preferred that he run,” David Axelrod, a senior adviser to the president, said in an interview on Friday. “But there are large numbers of incumbents retiring on both sides, and my understanding is that he’s been planning this for some time.”

Senior Democratic officials said there were signs that Mr. Stupak, even as he negotiated the health care compromise, was considering not running for re-election. He had not raised much money and had done little campaigning.

Though Mr. Stupak’s loudest critics, by Friday, were Republicans, Tea Party supporters and abortion opponents, he had, over many chapters of a battering health care fight, angered a broad array of groups, even some who had been political allies. To hear some tell it, he had managed, by the end of his evolution on health care, to make everyone mad at some point or another.

Last fall, Mr. Stupak successfully pressed for an amendment to a House version of the health care bill adding tight restrictions on abortion coverage. Speaker Nancy Pelosi, seen as a champion of abortion rights, went along, stunning some abortion rights groups and stirring their anger against Mr. Stupak. (National abortion rights advocates lined up behind the sole Democratic candidate, an abortion rights supporter herself who had announced well before Friday that she would challenge Mr. Stupak.)

Even when the Senate seemed to resolve the abortion issue in its health care bill, Mr. Stupak said the language was insufficient, irritating his colleagues. As the debate unfolded, he sided with Roman Catholic bishops who opposed the Senate language, rather than with nuns, who supported it. He said he did not listen to the nuns when it came to writing legislative language related to abortion.

But in the final hours before the last vote in March, Mr. Stupak and fellow anti-abortion Democrats supported the bill after President Obama agreed to sign an executive order outlining prohibitions against the use of federal money for abortion. At a news conference announcing his deal with the White House, Mr. Stupak said the bishops were unrealistic not to accept the compromise.