Gabrielle Giffords, 40, became the first Jewish woman to be elected to Congress from Arizona in 2006 after campaigning on a conservative platform that stressed US-Mexico border security and fiscal responsibility.
But she voted for US President Barack Obama's landmark health care reform bill last year despite fierce opposition from Republicans and protests from Americans across the country at raucous town-hall meetings.
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It was unclear whether there was any political motive behind the attack, but in recent years Giffords had struggled to find the middle ground in an increasingly divided and angry American electorate.
She had held more than 20 "Congress on Your Corner" events like the one on Saturday since taking office in January 2007, meeting and greeting hundreds of people from the 8th district in the southeastern corner of the state along the Mexico border.
She had pushed for more troops to guard the US-Mexico border and favoured a path to legalization that would require illegal immigrants to pass a criminal background check, pay back taxes and learn English.
But she has distanced herself from a controversial immigration bill signed into law by Governor Jan Brewer last April that made it a crime to be in the state without proper immigration documents.
She drew the ire of conservatives by voting in favour of the health care bill in March 2010, and within hours of the vote vandals broke the windows of her office, the Arizona Daily Star reported at the time.
Palin offers condolences
Top Republicans and Democrats immediately condemned the shooting spree, and Conservative standard-bearer Sarah Palin offered her "sincere condolences" in a message posted on her Facebook page.
"We all pray for the victims and their families, and for peace and justice," she said on the social networking site.
Shortly after the March 2010 health care vote Giffords's name had appeared on a graphic posted on Palin's Facebook page that showed a map of the United States with gun sights on the districts of Democrats who had backed the health bill.
It urged voters to support Republicans in the upcoming congressional elections with the slogan: "It's time to take a stand."
At the time Palin was pushing back against the notion that she had encouraged gun violence by exhorting supporters of the conservative Tea Party movement: "don't retreat, reload," insisting it was just a metaphor for the upcoming elections.
Narrow victory
Giffords went on to win a narrow victory against Republican Jesse Kelly in the conservative district by emphasizing her membership in the Blue Dog Coalition of centrist Democrats who support fiscally conservative, pro-business policies.
"I'm not elated to be in the minority," she said after the final results were announced, showing that Republicans had won back control of the House of Representatives and eroded the Democratic majority in the Senate.
"But my job isn't to be in the majority. It's to do the work, and I'm looking forward to doing that starting next week."
Giffords had served on the House's Armed Services Committee, Foreign Affairs Committee and Science and Technology Committee.
She is married to NASA astronaut Mark Kelly, with whom she has two children.
Kelly was to lead the STS-134 Endeavour mission to the international space station in April, where he was to rendezvous with his twin brother Scott, another astronaut who has been at the station since October.
