Take a trip back down memory lane.  Skip over Bush I & 2.  Pass Clinton, Reagan, Carter, and Ford.  Now we reach a fair comparison in Richard Milhous Nixon.  The Select Committee led by Sam Ervin & the Special Prosecutor's office received threats from the White House of executive privilege.  The efforts to keep witnesses from appearing failed.  

Should this president try to prevent Comey from testifying then the call for impeachment will begin to fill the halls of Congress, from both parties.

Seems very strange that just three months ago this administration wanted to see all the facts come out, and wanted an end to the investigation.