“There are tons of people like this,” said Shannon McGregor, an assistant professor of communication at the University of Utah who studies social media. “He took these memes to their most violent extreme, but this is a pretty big world on social media.”

The genesis of Mr. Sayoc’s partisan awakening may never be known, but hints of it first appeared on his Facebook feed in early 2016, as the primary season for the presidential election was starting.

That February, he posted a link to a conspiracy theory video on YouTube titled, “Is Barack Obama THE ANTICHRIST — 100% PROOF Is There!” Days later, he posted a second YouTube video, “Satan Sent Obama to Destroy America,” and a clip featuring Sean Hannity, the Fox News host, which was called, “MUST HEAR: Sean Exposes Illegal Immigrant Crime Stats.” He posted several anti-Obama videos multiple times on his feed, interspersed with stories about personal finance and his favorite soccer players.

By the summer, Mr. Sayoc’s social media activity was all politics, all the time.

On Facebook, he posted stories from Infowars, World Net Daily, Breitbart and other right-wing websites. His posts, which rarely included commentary apart from the links, showed a fascination with Islamist terrorism, illegal immigration and anti-Clinton conspiracy theories. (On one post, a YouTube link, he wrote: “The Clinton have funneled two billion dollars through Clinton foundation.”)

In October, a month before Mr. Trump’s election, Mr. Sayoc posted a series of photos of himself at a Trump campaign rally, watching from the crowd in a red “Make America Great Again” hat.

Despite his prolific posting, Mr. Sayoc does not appear to have gained a wide audience. His Twitter account was followed by fewer than 1,200 people as of Friday morning, and although he had nearly 3,000 friends on Facebook, many of his posts were never commented on or shared.

Image Mr. Sayoc became a vocal supporter of President Trump.

He did, however, display an unusual tendency to post stories and images over and over again, sometimes dozens of times.