That phrase didn't refer to "white America"-- it referred to white supremacists.
The problem was that Clinton rankly speculated that half of Trump's supporters were made up of white supremacists, in a futile effort to divide his base. And later in that same paragraph she said this:
"He has given voice to their websites that used to only have 11,000 people -- now 11 million."
Even if Clinton's statistics were even close to correct, it was a counterproductive statement. Trump voters who were white supremacists were energized by being attacked directly by Clinton. Trump voters who were not white supremacists were energized by the likelihood that Clinton was exaggerating the numbers to spread FUD and misdirect from her own historically low polling numbers as a presidential candidate. And Bernie supporters who were on the fence wondered why Clinton was name-calling Trump supporters when a large part of Bernie's appeal had been the exact opposite kind of behavior-- instead employing care and caution when referring to the grievances of Trump supporters-- as a primary candidate.
But more importantly, presidential candidates don't just improvise new phrases like that in the middle of speeches. Has there been anything written about who authored that phrase for Clinton and why they did it?
Edit: clarification