Nailing defendants for obstruction of justice rather than an underlying crime is a favourite tool of federal prosecutors.

ECONOMIST: Better late than never? Tell that to Arthur Andersen

These commentators and members--who are convinced, perhaps, that the President committed perjury and obstruction of justice, which, as classes of crime, are high Crimes and Misdemeanors--may nevertheless vote not to convict because they believe that removal from office is unwarranted for this perjury and this obstruction of justice.

CNN: Sen. Gorton's closed-door impeachment statement

Although no person has been convicted and removed for obstruction of justice, the nature and gravity of this crime, punished more harshly under our laws than bribery, clearly is also a sufficient cause for conviction and removal.

CNN: Sen. Gorton's closed-door impeachment statement