that is the question that the white house is wrangling with over the al gonzales/karl rove flap. i am not a lawyer and i am not a constitutional law scholar. i don't know all of the precedents and rules and whatnot. i simply have my well worn copy of the united states of america's constitution- and in there it says that congress has the right to information from the executive branch.
contrary to popular belief, there is no executive privilege in the constitution- presidents have implied that there is such a thing based on interpretation of 'the separation of powers' that is in the constitution. this is often litigated because it is open to interpretation. the common notion is that presidential advisers and who not would not advise and speak freely to the executive if he/she thought that they would be hauled in front of a congressional committee.
my thought is- don't do stuff that will get you hauled in front of a congressional committee- but i realize that partisan politics come into play. there is a great summary of this issue here.
the big issue that is getting lost here is that it is highly unusual for a lame duck president's justice department to fire folks this late in the game- because usually he already has appointed his own minions. comparisons to clinton coming into the white house and clearing folks out are like apples and oranges because an incoming first term president wants to put in his own folk. couple this with the fact that this admin orchestrated- and got from the last congress- a provision in the patriot act renewal that the attorney general can appoint who he/she wants without congressional oversight- well- i think that this congress has a duty to oversee this nonsense. the cheney admin has a track record of hiring and keeping loyal folks- not folks who are interested in doing what is best for the country or even doing the job for which they are hired- competently. it has been strictly a power grab by cheney et al from day 1- and they are just pissed because they are being checked. get over it. and go and re-read the constitution. separation of powers does not mean the executive gets its way all of the time.