a felony `` corruptly or by threats or force ... -LSB- to -RSB- influenc -LSB- e -RSB- , obstruc -LSB- t -RSB- , or imped -LSB- e -RSB- , or endeavo -LSB- r -RSB- to influence , obstruct , or impede , the due administration of justice , '' 18 U. S. C. § 1503 -LRB- a -RRB-