former federal bureau investigation director jam comey testify june congressional committee investigate russia allege meddle 2016 u.s. presidential election comey expect tell senate intelligence committee president donald trump ask drop investigation former white house national security adviser michael flynn tie russia trump fire comey may 9. legal experts say trump could invoke doctrine call executive privilege try stop comey testify maneuver would draw backlash could challenge court say follow describe executive privilege work could backfire trump invoke executive privilege executive privilege legal doctrine allow president withhold information government branch u.s. supreme court rule 1974 u.s. nixon executive privilege use limit circumstances protect national security preserve confidentiality sensitive communications within executive branch would trump invoke privilege block comey testimony invoke executive privilege president typically obtain write memorandum justify decision office legal counsel division department justice white house counsel often informally involve decision invoke privilege president direct current former government officials divulge information presidents invoke executive privilege past term coin 1950s presidents invoke version say mark rozell professor government george mason university president barack obama assert privilege 2012 block congress see document relate investigation fast furious botch gunrunning operation conduct bureau alcohol tobacco firearms explosives odds trump would legally succeed block comey testimony legal experts say trump would face uphill climb assert executive privilege stop comey testify congressional committee trump likely would argue comey testimony involve confidential conversations matter national security claim would undercut fact president publicly discuss tweet conversations comey say rozell trump face another hurdle try block comey testimony trump pressure comey drop flynn investigation comey expect testify trump may engage obstruction justice accord lawyers executive privilege use cover government misconduct say andrew wright professor savannah law school trump invoke privilege could comey disregard typically president use executive privilege prevent government employees release information comey private citizen worry lose job comply rozell say know legal sanction ignore assertion executive privilege would unprecedented assertion privilege ignore could trump challenge court assert executive privilege president congress typically hammer compromise disagree whether privilege assert say rozell instance executive privilege challenge court obama cite privilege block release document relate fast furious congress sue ask judge direct then-attorney general eric holder comply subpoena judge rule last year justice department public disclosures controversy undercut president executive privilege claim say justice department already publicly reveal much information say keep private would procedurally complicate challenge decision trump block comey testimony first congress would issue subpoena require comey testify congress would find comey contempt refuse congress could ask federal judge force comey comply subpoena litigation would lead rule whether trump lawfully invoke executive privilege path judicial review lot things would take place wright say political risk trump try assert privilege trump would surely face public criticism try stop comey testimony rozell say critics could claim trump use privilege thwart question potential tie russia flynn russia allege influence election rub executive privilege make look like something hide rozell say factbox could trump use executive privilege block comey testimony