The showdown between the White House and key congressional Republicans over the administration's response to a controversial program that allowed guns into the hands of Mexican drug cartels kicked into high gear Wednesday.

President Obama granted Attorney General Eric Holder executive privilege on the matter, while the head of the House panel overseeing government conduct moved forward with proceedings to hold the nation's top law enforcement official in contempt of Congress.

By asserting executive privilege, the Justice Department can withhold documents from Congress, even if Congress has issued a subpoena, as the House Oversight and Government Reform Committee has.