WASHINGTON — Rep. Charles Schumer (D-N.Y.) has asked the Justice Dept. and the Federal Trade Commission to investigate Cablevision Systems Corp.’s pending deal to acquire Madison Square Garden, along with the New York Knicks, the New York Rangers and the MSG Sports Network.Cablevision already owns 50% of MSG and plans to purchase the remaining 50% from ITT for $650 million. In letters to the two agencies, Schumer claims the deal violates antitrust laws and will result in higher cable rates because it will give Cablevision a lock on several New York sports teams. Cablevision also owns the broadcast rights to the Rangers, Yankees, Mets and Islanders. A spokesman for Cablevision said the company had offered to sit down with Schumer to “clear up the many misconceptions,” but that so far, Schumer has not acknowledged the invitation. The spokesman declined to address Schumer’s allegations, including his claim that the deal will result in higher cable rates.