Democratic Representative Gabrielle Giffords, 40, was shot at close range during a public meeting in Tucson, Arizona.
The doctor treating her said he was “very optimistic about her recovery”. The dead included a nine-year-old girl and a federal judge.
A suspect named by US media as Arizona resident Jared Loughner, 22, was arrested after the incident.
“[The suspect] has kind of a troubled past, and we’re not convinced that he acted alone,” -said Pima County Sheriff Clarence Dupnik.-“There is some reason to believe that he came to this location with another individual.”
The attacker struck as Ms Giffords held one of her regular open-invitation meetings – which she called “Congress on your corner” – with her constituents outside a supermarket in Tucson.
The dead judge was named by officials as John Roll, who had spent more than 40 year on the federal bench. It is not thought that he was deliberately targeted.
Mr Obama urged the nation to “come together, and support each other”.
“I ask all Americans to join me and Michelle in keeping all the victims and their families, including Gabby, in our thoughts and prayers,” – he said.
He described Ms Giffords as a personal friend and an “extraordinary public servant”.
Ms Giffords, who represents the eighth district of Arizona in the House of Representatives, is married to space shuttle astronaut Mark Kelly. She has served on several congressional committees.
The House of Representatives majority leader Eric Cantor announced that all of next week’s legislative debates would be postponed to allow politicians to “take whatever actions may be necessary in light of today’s tragedy”.