A former Democratic congressional staffer pleaded guilty Friday to federal charges related to his leaking personal information about Republican senators and threatening a witness who caught him in the act. Jackson A. Cosko, 27, of D.C., copped to five offenses, including computer fraud, making public restricted personal information, witness tampering and obstruction of justice. He faces nearly five years in prison when he is sentenced June 13. Cosko admitted to “doxing,” the act of posting personal information about another individual online. He said the goal was to “threaten and intimidate” the senators and their families. He posted the personal addresses...