The subreddit is intentionally insular, but widely read, ranking among the most active on the site. According to Reddit’s own measurements, it produced nearly 52 million page views in March. In the same month, the largest subreddit supporting Mr. Sanders, who has visited the site to answer questions, and whose campaign maintains an active presence there, drew almost 36 million, despite having more subscribers.

In subreddits supporting other candidates, users promote favorable stories, feud with foes and rally support through phone-banking or “Facebanking” — campaigning to Facebook friends. On The Donald, the message is relentless — as are the insults. Opponents are referred to as “cucks,” which is short for “cuckservative,” as in “cuckold” — now used as a derisive term for liberals and moderate Republicans recently popularized by far-right online commentators and white nationalists, according to the Southern Poverty Law Center. The group shares content and tone with parts of 4chan, the infamous and anonymous message board that traffics in shock, and where Mr. Trump — who regularly scorns “political correctness” — has found substantial, if oblique, support.

Visitors to the group will find a cascade of offensive postings. Some members share open antipathy toward Muslims, sling insults with relish and mock anyone who takes umbrage.

“We have a relationship with 4chan,” said one Reddit moderator, who spoke on the condition that he not be identified out of fear, he said, of harassment. “It’s a little bit scary to some people, the way they talk.” (Moderators, like all Reddit users, are listed only by self-selected handles).

To charges of racism, the moderator, who said he was a 32-year-old technology worker, said that “white nationalists” had been banned from the group. “We kicked them out before it became an issue for Trump,” he said in a phone interview. Members respond to accusations of bigotry with defiant claims of persecution at the hands of critics. It is an article of faith among posters that anti-racists are the real bigots, feminists are the actual sexists, and progressive politics are, in effect, regressive.

Support for Mr. Trump on 4chan and similar sites started off as a manic elation and disbelief at the spectacle of it all — he’s behaving like us! On Reddit at least, that elation has turned into real support. In between meme photos and lengthy threads of mockery, users also post calls to action that offer tangible benefits to Mr. Trump. They remind one another of voter registration deadlines and share rhetorical strategies for dealing with opponents. The group maintains a delegate counter, links to pro-Trump videos and articles, and provides materials instructing new visitors on how to help with the campaign.

Recently, the group has focused on holding question-and-answer sessions — known on Reddit as AMAs, for Ask Me Anything — with figures including the conservative commentator Ann Coulter, the immigration activist Roy Beck and Helmut Norpoth, a political-science professor at Stony Brook University whose election model is favorable to Mr. Trump. (Mr. Norpoth, who did not express support for any candidate, said in an interview that he had been invited to participate by a student, but was unaware that he was speaking to a Donald Trump group. “It took me a little bit to find my bearings,” he said.)