The embarrassing disclosure that “60 Minutes” featured what appear to be forged memos in its coverage of President Bush’s National Guard Service is a blow to the reputations of CBS News and to the distinguished, four-decade career of Dan Rather.But the ripple effects could be far more damaging for the media, amid the customary charges of “liberal bias,” if it results in a chilling effect on efforts by news orgs to break aggressive, politically charged stories in a hard-fought election season. This election has been clouded by ongoing questions about the accountability of the White House on matters like Iraq, terrorism and the economy. But those questions have been overshaded by more superficial media controversies, from the veracity of the Swift Boat ads to Michael Moore’s “Fahrenheit 9/11.” It is the responsibility of journalistic institutions at moments like this to dig beneath such surface controversies, to remain unaffected by partisan spin and to frame the debate around key issues that partisans on both sides have sought to obfuscate. CBS News has been lauded for taking the lead on such investigative work. It was the first broadcast net to report on the abuse of prisoners at Iraq’s Abu Ghraib prison. Fending off conservative charges that the media was exploiting the prison scandal to discredit the Bush administration, “60 Minutes II” aired graphic photos that prompted an investigation by the U.S. Army. CBS shouldn’t be faulted for exploring the president’s National Guard Service, though like challenger John Kerry’s Vietnam-era record, that seems at best peripherally germane to the pressing issues of the day. The real damage in botching the story, however, would be if the ammunition with which CBS has armed its critics blunts the willingness of so-called mainstream media outlets to ask tough questions of the current administration.