Humans of New York blogger Brandon Stanton is known for giving a voice to the voiceless with his long-captioned social media posts.

But on Monday, he did something that he doesn't do often: he used his own voice.

In what he calls an "open letter to Donald Trump," the blogger breaks out his political side in a Facebook post -- just days after massive protests led Trump to cancel a rally in Chicago.

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"For the longest time I felt a reluctance to speak out strongly because I didn't want to involve myself in politics," Stanton told CBS News on Monday. "But Trump's appeal to hatred and violence has grown to the point where I think every person of influence needs to make a decision. Do you maintain a political silence? Or do you take a moral stand?"

In his letter, which was shared by nearly 500,000 people in three hours, Stanton said that now is the time to speak up.

"Because along with millions of Americans, I've come to realize that opposing you is no longer a political decision," Stanton directed at Trump. "It is a moral one."

Stanton criticized the Republican front-runner for retweeting what he calls racist images and lies.

In January, Trump was under fire for retweeting a post from a white nationalist's Twitter account (@WhiteGenocideTM) that mocked former Florida Gov. Jeb Bush.

Screenshot shows controversial picture Donald Trump retweeted in January. Twitter/@realDonaldTrump

A month later, the GOP candidate was caught retweeting a quote from Italian fascist dictator Benito Mussolini, which turned out to be an elaborate set-up by Gawker.

When asked about the tweet on NBC's "Meet the Press," Trump said he realized it was a Mussolini quote when he retweeted it; he thought it was an "interesting quote."

Stanton is also upset that Trump has promised to "'pay the legal fees' of those who commit violence on [his] behalf," he wrote, referring to a man who punched a protester at a Trump rally. "I've watched you joyfully encourage violence."

Toward the end of his more than 300-word post, Stanton turned his attention to Muslims and the refugee crisis -- a topic Stanton is very passionate about.

Over the past two years, Stanton has been sharing stories and interviews with hundreds of Muslims, as well as Syrian and Iraqi refugees "across seven different countries."

"And I can confirm -- the hateful one is you," Stanton stated.

14-year-old girl named Aya featured on Humans of New York in December. Facebook/Humans of New York

When Trump called for a "a total and complete shutdown" of Muslims entering the U.S. in early December, Stanton tried to draw attention to the situation on his website.

He shared the story of Aya, a 14-year-old Muslim girl who fled the war in Syria and dreamed of coming to America.

The teen's application for resettlement in the U.S. was rejected. So, Stanton started an online petition to bring her to the country, which has garnered nearly 1.1 million supporters.

"There is so much fear in the world right now," Stanton wrote on Dec. 17 after spreading Aya's message. "And politicians know that they must win the votes of frightened people."

That's why Trump shouldn't be considered a "unifier" or "presidential," Stanton said in his most recent post on the issue.

"You are a man who has encouraged prejudice and violence in the pursuit of personal power," he said. "And though your words will no doubt change over the next few months, you will always remain who you are."

While thousands praised Stanton for speaking out on the topic, others criticized the popular blogger for sparking a political discussion.

"Another opinion and trashing from the left," one user wrote. "Don't believe all you hear."

"I guess it's one hater to another then," another commented. "Talk about fueling fire and spewing an opinion based on inflated half truths."

But with more than 70,000 likes, the top comment on the post questioned Trump supporters.

"From every other part of the world, we are wondering how he is even getting so many votes!" a social media user exclaimed.

"Thank you, Brandon. We need more LOVE and KINDNESS in this world, not hate and fear," another user thanked the man.