WASHINGTON — Republicans on Tuesday sought to intensify political pressure on Attorney General Eric H. Holder Jr. over a disputed investigation into a gun trafficking network based in Phoenix, accusing him of misleading Congress. Representative Lamar Smith, Republican of Texas and chairman of the House Judiciary Committee, sent a letter to President Obama asking him to appoint a special counsel to investigate whether Mr. Holder committed perjury in testimony about the investigation, known as Operation Fast and Furious, at a hearing in May. Mr. Holder testified that he had only recently heard about the matter. But Mr. Smith pointed to...