Washington — For months, Representative Gabrielle Giffords, who was shot in the head in an assassination attempt last year, signaled that returning to Congress, something she desperately longed to do, was in the realm of the possible.
She listened pensively as her friend, Senator Kirsten Gillibrand of New York, briefed her on the conflict in Libya, and she expressed in clipped phrases her views on the matter. She cast a vote to raise the nation’s debt ceiling. Her Congressional aides continued to churn out news releases outlining her positions and hold community meetings, and she and her husband gave an interview to ABC News in which she demonstrated her improving ability to speak.
But Ms. Giffords, a moderate Democrat from Arizona whose remarkable comeback stirred the nation, decided in recent days that she could not continue her recovery and still serve as a member of Congress. On Sunday, she announced that she would step down.
Ms. Giffords’s decision shook up the race for her seat representing Arizona’s Eighth District. She barely fought off her Republican challenger in 2010 but was expected to be a shoo-in for re-election if she had decided to run this year.
“She could have definitely done it,” Ms. Gillibrand said. “But I think she made a realization, if she really wanted to focus on the recovery, she shouldn’t.”
In a moving video released online on Sunday afternoon, Ms. Giffords, in a halting voice, explained to her constituents: “I don’t remember much from that horrible day, but I will never forget the trust you placed in me to be your voice. Thank you for your prayers and for giving me time to recover. I have more work to do on my recovery, so to do what is best for Arizona, I will step down this week.”
But Ms. Giffords hinted at a potential return to elective office. “I will return, and we will work together for Arizona and this great country,” she said.
On Jan. 8, 2011, Ms. Giffords, who is in her third term, was one of 19 people shot at a meet-and-greet political event outside a grocery store in her hometown of Tucson. Six people died, including a 9-year-old girl, Christina-Taylor Green, and a federal judge, John M. Roll.
The remainder of Ms. Giffords’s term will be filled by the winner of a special election, to be held on a date determined by Gov. Jan Brewer, a Republican. In November, the district will be redrawn in a way that further favors Democrats, which may scare away some Republicans.
Democrats, fearing the loss of the seat, had hoped that Ms. Giffords’s husband, Capt. Mark E. Kelly, a retired astronaut, would run in her place.
Michael McNulty, the chairman of the Giffords for Congress campaign, said he talked with Mr. Kelly on Saturday. When asked if he was interested in running, “he said no, simply no,” Mr. McNulty said.
Ms. Giffords’s decision came after intensive discussions with her political advisers but ultimately boiled down to a personal choice, those close to her said.
Democrats in Arizona, who had been waiting to see what Ms. Giffords would decide, were informed of the congresswoman’s resignation just before she posted her video statement online on Sunday. Until then, her adviser had said that the only deadline she faced about her political future was May 30, the deadline for submitting her nominating petitions for re-election.
“There was definitely no pressure on her,” Ms. Gillibrand said. “I think everyone in the state wanted Gabby to serve and was letting her decide her response accordingly.”
While Ms. Giffords’s ability to understand what people say to her is strong, she does not have the language skills to express her responses. “She can get across how she feels and what her views are,” Ms. Gillibrand said, but her ability to communicate effectively is hindered “because she can’t talk in paragraphs. That is the development she wants to focus on.”
Under Arizona law, a governor has 72 hours from the day the vacancy is declared to name the date for a special election; the primary must take place 80 to 90 days from the date of the vacancy, with a general election 50 to 60 days after that.
Jeff Rogers, the chairman of the Democratic Party in Pima County, which includes Tucson, said money could play a significant role, especially in the special election, which tends to draw fewer voters.
“There’s no odds-on favorite, Democrat or Republican,” he said. “But if she supports anyone, it’s a leg up.”
Several Democrats who hold statewide posts are expected to test the waters, including Matt Heinz, a doctor and member of the Legislature; Paula Aboud, a state senator; and Steve Farley, assistant minority leader in the State House and a friend of Ms. Giffords and Mr. Kelly.
Frank Antenori, a Republican state senator from what is now the southern part of the district who is planning to run for the seat, seemed thrown by the new development.
“I’m still trying to sort this out,” said Mr. Antenori, who called a strategy session of his advisers on Sunday night to discuss the matter.
“I had heard that an announcement was coming, but most people thought that she wouldn’t run again, not that she’d step down right away,” he said. “I expect a lot of national attention and a lot of national money in this race.”
Ms. Giffords has a healthy campaign treasury, with just shy of $880,000 on hand as of the end of September; other potential Democratic candidates will start with far less in the way of money and organization.
Ms. Giffords’s office said she planned to return to the supermarket parking lot in Tucson where she was shot for a private event, which will include some of the people who were at the meet-and-greet last year.
On Sunday, Mr. Obama released a statement that read in part: “Gabby Giffords embodies the very best of what public service should be. She’s universally admired for qualities that transcend party or ideology — a dedication to fairness, a willingness to listen to different ideas, and a tireless commitment to the work of perfecting our union.”
John A. Boehner, a Republican who became House speaker only days before the shooting, said in a statement on Sunday: “I salute Representative Giffords for her service, and for the courage and perseverance she has shown in the face of tragedy. She will be missed.”
Reporting was contributed by Ken Belson and Ford Burkhart from Phoenix, Marc Lacey from New York and Marisa Gerber from Tubac, Ariz.
This article, "For Giffords, House Comeback Is One Too Many," originally appeared at The New York Times.
This article has been revised to reflect the following correction:
Correction: January 23, 2012
An earlier version of this article incorrectly stated the television network to which Representative Gabrielle Giffords and her husband, Capt. Mark E. Kelly, gave an interview. They appeared on a television special on ABC, not on CBS News’s “60 Minutes.”