Giffords represents Arizona's 8th Congressional District, "a diverse area that covers 9,000 square miles including a 114 mile border with Mexico," according to her website.

She has previously has security issues in the Southern Arizona district, and her office was vandalized after she voted in favor of the health-care overhaul in March.

Her last tweet before the shooting: "My 1st Congress on Your Corner starts now. Please stop by to let me know what is on your mind or tweet me later."

Republican House Speaker John Boehner issued a statement condemning the shooting. "I am horrified by the senseless attack on Congresswoman Gabrielle Giffords and members of her staff," he said. "An attack on one who serves is an attack on all who serve. Acts and threats of violence against public officials have no place in our society. Our prayers are with Congresswoman Giffords, her staff, all who were injured, and their families. This is a sad day for our country."

(See a picture of Giffords with Speaker Boehner on Wednesday, from her Twitter feed.)

President Obama also condemned the attack, which he called "an unspeakable tragedy."

"While we are continuing to receive information, we know that some have passed away, and that Representative Giffords is gravely wounded," the president said.

"We do not yet have all the answers. What we do know is that such a senseless and terrible act of violence has no place in a free society," he continued. "I ask all Americans to join me and Michelle in keeping Representative Giffords, the victims of this tragedy, and their families in our prayers."

Giffords narrowly won reelection in 2010 against tea party-backed candidate Jesse Kelly. She also was one of the Democrats targeted by former GOP vice presidential nominee Sarah Palin in an online map that controversially placed rifle scope marks over the districts of Democrats.

"Sarah Palin ... has the crosshairs of a gun sight over our district and when people do that, they've gotta realize there are consequences to that action," Giffords told MSNBC in March.

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Palin issued a statement on Facebook Saturday afternoon offering "My sincere condolences ... to the family of Rep. Gabrielle Giffords and the other victims of today's tragic shooting in Arizona. On behalf of Todd and my family, we all pray for the victims and their families, and for peace and justice."

Giffords' father told The New York Post he suspected the attack was politically motivated:

The congresswoman's father Spencer Gifford, 75, was rushing to the hospital when asked if his 40-year-old daughter had any enemies. "Yeah," he told The Post. "The whole tea party."

In 2009, a tea party supporter brought a gun to a Giffords event, Gail Collins recounted in a column looking at the anti-health-care overhaul protests: