Our cases establish a clear line between a tax and a penalty: “[A] tax is an enforced contribution to provide for the support of government; a penalty … is an exaction imposed by statute as punishment for an unlawful act.” United States v.Reorganized CF&I Fabricators of Utah, Inc., 518 U. S. 213, 224 (1996) (quoting United States v.La Franca, 282 U. S. 568, 572 (1931)). In a few cases, this Court has held that a “tax” imposed upon private conduct was so onerous as to be in effect a penalty. But we have never held—never—that a penalty imposed for violation of the law was so trivial as to be in effect a tax. We have never held that any exaction imposed for violation of the law is an exercise of Congress’ taxing power—even when the statute calls it a tax, much less when (as here) the statute repeatedly calls it a penalty.