Loretta Sanchez, the current representative for California's 46th congressional district, had announced her candidacy only on May 14.
Sanchez, a long-time politician who has served in Congress for ten consecutive terms, is challenging Indian-American and California Attorney General Kamala Harris for the prestigious post in the US Senate in 2016.
The online video shows Sanchez tapping her hand over her open mouth and making a typical native American 'war cry' while speaking to a group of delegates on Saturday.
Yesterday, Sanchez apologised at a state Democratic convention in Anaheim, saying "in this crazy and exciting rush of meetings yesterday, I said something offensive and for that, I sincerely apologise."
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Earlier Sanchez told the Bee that she "got a call from somebody over the phone and he said, 'I want to talk to you about having help from the Indian community,' and I thought he meant the American Indian community, in the sense of the Native American Indian community."
The video was first reported by the local Sacramento Bee.
"I don't know what to say to that...Except that...That's shocking. That's shocking," Harris said.