WASHINGTON – Rep. Gabrielle Giffords of Arizona announced Sunday that she intends to resign from Congress this week to concentrate on recovering from wounds suffered in an assassination attempt a little more than a year ago that shook the country.

“I don’t remember much from that horrible day, but I will never forget the trust you placed in me to be your voice,” the Democratic lawmaker said on a video posted without prior notice on her Facebook page.

“I’m getting better. Every day my spirit is high,” she said. “I have more work to do on my recovery. So to do what’s best for Arizona, I will step down this week.”

Giffords was shot in the head and grievously wounded last January as she was meeting with constituents outside a supermarket in Tucson, Ariz. Her progress had seemed remarkable, to the point that she was able to walk dramatically into the House chamber last August to cast a vote.

Her shooting prompted an agonizing national debate about super-charged rhetoric in political campaigns, although the man charged in the shooting later turned out to be mentally ill.

In Washington, members of Congress were told to pay more attention to their physical security. Legislation was introduced to ban high-capacity ammunition clips, although it never advanced.

Under state law, Arizona Gov. Jan Brewer must call a special election to fill out the remainder of Giffords’ term, which concludes at the end of 2012.

President Obama on Sunday called Giffords “the very best of what public service should be.”

“Gabby’s cheerful presence will be missed in Washington,” Obama said. “But she will remain an inspiration to all whose lives she touched — myself included. And I’m confident that we haven’t seen the last of this extraordinary American.”

House Speaker John Boehner, R-Ohio, said he saluted Giffords “for her service and for the courage and perseverance she has shown in the face of tragedy.”

House Democratic leader Nancy Pelosi of California said that “since the tragic events one year ago, Gabby has been an inspiring symbol of determination and courage to millions of Americans.”

Maine Rep. Chellie Pingree, D-1st District, said she will miss Giffords’ presence on Capitol Hill.

“As a colleague I will miss Gabby’s courage, her compassion and her conviction in Congress,” Pingree said in a statement Sunday. “As a friend, I know she is doing what is right for her and her family.”

Democratic officials held out hope for months that Giffords might recover sufficiently to run for re-election or even become a candidate to replace retiring Republican Sen. Jon Kyl.

The Jan. 8, 2011, shooting left six people dead, a federal judge and a Giffords aide among them. Twelve others were wounded.

Jared Lee Loughner, 23, has pleaded not guilty to 49 charges in the shooting. He has been diagnosed with bipolar disorder, and is being forcibly medicated at a Missouri prison facility in an effort by authorities to make him mentally ready for trial.

In the months since she was shot, Giffords, 41, has been treated in Houston as well as Arizona as she re-learned how to walk and speak.

She made a dramatic appearance on the House floor Aug. 2, when she unexpectedly walked in to vote for an increase in the debt limit. Lawmakers from both parties cheered her presence, and she was enveloped in hugs.