The subreddit has been fertile ground for pro-Trump images and videos that spread to other social media platforms, with support from within the president’s circle. In fact, days after being named digital director of the Trump campaign in 2016, Brad Parscale, now the campaign manager of the president’s re-election campaign, tweeted that he visited The_Donald daily.

“It was clear that they had an audience with not only a great deal of Trump’s supporters but with Trump himself,” said Christine Lagorio-Chafkin, a senior writer at Inc. and author of “We Are the Nerds,” a book about Reddit.

In July 2016, Mr. Trump answered questions from users of the subreddit. Throughout the campaign and the early days of his presidency, Mr. Trump mined the community for memes he would later amplify on social media.

With hundreds of thousands of subscribers, The_Donald is home to a broad conversation ranging from discussion about the news to media criticism to jokes. But the community has also been known to spread extremist messages and conspiracy theories, including the debunked “Pizzagate” story, which centered on a belief that a Washington-area restaurant was secretly home to a child-abuse ring. The subreddit has also been criticized for allowing an undercurrent of racist, Islamophobic and anti-Semitic tropes and language.

The response by Reddit on Wednesday stopped short of an outright ban of the group, a step taken by administrators twice in 2017, when they cracked down on Nazi, white supremacist and far right subreddits and later expunged a forum about the subculture of “incels,” or involuntary celibate men, because of threats of violence against women.