Prosecutors and attorneys general however commit obstruction of justice when they fail to prosecute judges and other government officials for malfeasance misfeasance or nonfeasance in office. ~ WikipediaThe moment George Bush instructed the Justice Department to not "pursue contempt charges initiated by Congress against White House officials" he has committed a felony, Obstruction of Justice.
This alone is sufficient grounds for impeachment.
Additional thought: Congress should call the Federal district attorney to testify under oath to discover one thing - who (if anyone) has ordered him to not impanel as required by law a grand jury to investigate the "contempt of Congress" charges he has received? His testimony and any written orders to this effect is proof of felony actions by the source.