This subject comes up a lot here on Free Republic, but never is mentioned anywhere else that I have heard.
Realistically, there will be no challenge to the eligibility of someone born in America of non-citizen parents.
The notion that Natural Born Citizen status required both parents to be U.S. citizens at time of birth was a "Plan B" of sorts in the event the "Born in Kenya" conspiracy fell apart (not that even a Kenyan birth certificate would have changed things). It has never enjoyed support outside of a small fringe of anti-Obama activists (though I'm sure that if Donald Jr or Ivanka would run for president, CNN would take serious look at this).
Realistically, there will be no challenge to the eligibility of someone born in America of non-citizen parents.
The people arguing she's ineligible would not vote for her for any reason. There will not be a single voter that she loses over this.
Personally, I think the Democrats would be foolish to rally behind Kamala Harris for reasons that have nothing to do with questions of her eligibility. She's a loathsome, angry, scowling woman. Uninspiring and bereft of ideas, she would energize in opposition the same "Anybody but Hillary" voters who helped propel Donald Trump to victory. And she's got plenty of skeletons in her closet.