http://www.mediaite.com/tv/bernie-sanders-to-liberty-u-america-was-founded-on-racist-principles/
In a very narrow sense, yes of course, just like every other country in the world at the time, the US prevented all peoples from expressing themselves democratically and while the institution of slavery was protested, it was not outlawed.
That being said, it is incredibly naive and narrow-minded to hold the founding fathers to a higher standard that places them outside of the 18th century. The US constitution and its founding documents established and enshrined liberal principles that gave its citizens unparalleled rights at the time. Of course it was not perfect, but the wording of constitution was the basis upon which slavery was eventually outlawed and equality under the law restored to the nation.
I would have expected more from Sanders to be honest.
I didn't realize he was running to be President of every country in the world?
If you actually listened to the full exchange/original question, then you would know that it was a response to "what would you do to combat racism?". Obviously to discuss where we are currently, one must talk about from where we came. Therefore, understanding the current situation involves addressing the fact that we were "founded on racist principles".
And let's be real here: y'all saying that "I don't know if I'm going to support Sanders now" were never
actually going to end up casting a vote for him before this (with the exception of you White National Socialists, perhaps - show of hands?). If you can't bear to hear within relevant contexts about our racist past without getting your panties in a wad, then you know what I'm going to say.