Under the Unitary Executive theory of US government,  the President is responsible for everything that happens in the executive branch, and has unilateral authority to manage the affairs of the various departments and agencies, including the FBI.  The firing of Comey was well within the President's authority.  By firing the Director, Trump had no direct effect whatsoever on the actual Russia investigation.  To claim otherwise would be to claim that firing any official higher up the food chain than the direct leader of the investigation would also be obstruction.  That would include the various Deputy Directors of the FBI,  the Assistant Attorney's General,  the Deputy Attorney General and the AG himself.  Do the Dems think that all those individuals are sacrosanct?