The Stevens dissent in Citizens United explains:

If there was ever any significant uncertainty about what counts as the functional equivalent of express advocacy, there has been little doubt about what counts as express advocacy since the “magic words” test of Buckley v. Valeo, 424 U. S. 1, 44, n. 52 (1976) (per curiam).

Footnote 52, of course, described “express words of advocacy of election or defeat, such as ‘vote for,’ ‘elect,’ ‘support,’ ‘cast your ballot for,’ ‘Smith for Congress,’ ‘vote against,’ ‘defeat,’ ‘reject.'”

Prof. Allison Hayward notes footnote 8 of the Stevens dissent in Citizens United in this passage at the Center for Competitive Politics blog:

But even those sympathetic to his overall argument may be shocked to learn:

. . .

— in footnote 8, contending that the “express advocacy” standard has always been clear and thus does not chill speech. I need not rehearse the history of express advocacy, Faucher, Furgatch, the MCFL rulemaking here. Yeah, those were the days, huh?  No issue at all where the express advocacy line should be.

After Citizens United, what’s “express advocacy”?