Once more the Court is required to resolve the conflicting constitutional claims of congressional power and of an individual's right to resist its exercise. The congressional power in question concerns the internal process of Congress in moving within its legislative domain; it involves the utilization of its committees to secure "testimony needed to enable it efficiently to exercise a legislative function belonging to it under the Constitution." McGrain v. Daugherty, 273 U. S. 135, 160. The power of inquiry has been employed by Congress throughout our history, over the whole range of the national interests concerning which Congress might legislate or decide upon due investigation not to legislate; it has similarly been utilized in determining what to appropriate from the national purse, or whether to appropriate. The scope of the power of inquiry, in short, is as penetrating and far-reaching as the potential power to enact and appropriate under the Constitution.