Stanton wrote that he has so far tried not be political, turning down requests to interview candidates in the 2016 race. But opposing Donald Trump, he said, is "not a political decision, but a moral one."
"I've watched you retweet racist images. I've watched you retweet racist lies. I've watched you take 48 hours to disavow white supremacy. I've watched you joyfully encourage violence, and promise to 'pay the legal fees' of those who commit violence on your behalf," Stanton wrote.
"I've watched you advocate the use of torture and the murder of terrorists' families. I've watched you gleefully tell stories of executing Muslims with bullets dipped in pig blood. I've watched you compare refugees to 'snakes', and claim that 'Islam hates us'."
The letter comes as authorities in North Carolina say they are looking at Trump's behaviour in relation to a violent altercation at one of his rallies last week.
The Sheriff's Office said that its investigators are continuing to look at the rally last week in Fayetteville, during which a protester was hit in the face while being escorted out.
"We are continuing to look at the totality of these circumstances ... including the potential of whether there was conduct on the part of Mr Trump or the Trump campaign which rose to the level of inciting a riot," it said. Trump's campaign manager, Corey Lewandowski, said: "They have not reached out to us at all."
Authorities have already charged a rally attendee with assault, disorderly conduct and communicating threats. At one point during the rally, Trump described a previous event in which a protester traded punches with his supporters.
Trump told the audience: "They started punching back. It was a beautiful thing."
