18 U.S.C. 1503(a) -- Obstruction of Justice
(a) Whoever corruptly... or by any threatening letter or communication, endeavors to influence, intimidate, or impede any ... officer in or of any court of the United States... in the discharge of his duty,... or by any threatening letter or communication, influences, obstructs, or impedes, or endeavors to influence, obstruct, or impede, the due administration of justice, shall be punished as provided in subsection (b)[.]
Publius appears to focus initially on the "endeavors to intimidate" section to get Domenici and Wilson, but then shifts to administration officials potentially firing attorneys to stop investigations.
I think there's another possible obstruction of justice. Domenici and Wilson might have valued a rushed indictment of Democrats prior to the November election more than they valued an actual conviction that might occur from an unrushed indictment afterwards. "Endeavoring to influence" DOJ to get a rushed and flawed indictment early on that impedes the prospect of conviction later because the indictment was brought before all the evidence could be gathered, sounds to me like obstruction of justice.
Of course the indictments weren't rushed in the end, but "endeavoring" is the basis of the crime - the attempt is enough.
In this view the key issue is the calls Domenici and Wilson made, but the eventual firing could be seen as part of the overall process to influence the system. When someone fails to obstruct justice as required, he must be fired. The firing, if intentionally connected to the failure to bring a rushed indictment, could then involve Bush Administration officials as also being part of the obstruction of justice, either directly or as accomplices after the fact.
Not my legal field, but could be interesting.