The man arrested by the authorities on Friday in connection with a series of bombs sent in the mail appears to be an outspoken supporter of President Trump, according to his posts on social media. The man attended a rally by Mr. Trump in West Palm Beach, Fla., on Oct. 13, 2016; the president’s inauguration on Jan. 20, 2017; and then another rally at Orlando Melbourne International Airport on Feb. 18, 2017. At the first rally, which took place weeks before Election Day, the suspect, Cesar Sayoc Jr., 56, of Aventura, Fla., recorded a selfie video. At the same rally,...