FORT LAUDERDALE, Fla.—The Florida man arrested Friday for allegedly mailing bombs to prominent Democrats was a former stripper and failed entrepreneur with a long criminal record who became an avid fan of President Trump in recent years.

Cesar Altieri Sayoc, a 56-year-old south Florida resident who was born in Brooklyn, N.Y., spent a lot of time researching politics, said Justin Humberger, who said he met the suspect about two years ago and recently tried to start a fish-farm business with him.

“Everything...