These are the kinds of people that we are dealing with. This is the kind of political movement Donald Trump is attempting to build. We cannot allow it to succeed.

This was not an isolated incident. It was yelled out multiple times by people in the crowd, repeatedly, you can check for yourself by re-watching the rally, or by glancing through comment threads from websites where people were watching the rally and searching for the term "white power". For example:

Anonymous 21 August 2015, 16:52
OH KEK SOMEONE SAID WHITE POWER
Anonymous 21 August 2015, 16:57
WHITE POWER
DID ANYONE HEAR THAT??

Anonymous 21 August 2015, 16:28
DID THAT GUY JUST YELL WHITE POWER????!!!
Anonymous 21 August 2015, 16:45
It's the third or fourth time I've heard it

Washington Post reporter Dave Weigel, who is shadowing the campaign sarcastically noted this morning that his interviews with citizens of Alabama are “going well so far,” with a local farmer telling him: “You probably think we’re prejudiced, but my whole life we had n***rs work for us in the field. And they were n**rs. My daddy called them n**rs. I’m not ignorant. That’s just the way I was raised. There’s black people and there’s n***rs. You live around here, you know the difference.”