If a judge overturns an incorrect precedent, he is not engaged in activism. If he strikes down an unconstitutional statute, he is not engaged in activism.
On the other hand, if a judge decides that Congress needs to exercise powers the Constitution reserves to the states, and then upholds legislation that previous courts have struck down, he is an activist (in the mode of Felix Frankfurter). The same is true if he upholds precedents he knows to be unfounded. Or if—as in the child rape case, or in Roe v. Wade—he strikes down laws simply because he dislikes them.
(In this essay Professor Gutzman expresses his support for the decision in Citizens United v. FEC.)
