The biggest cases of the Supreme Court’s term generally come down at the end of June. The campaign finance case probably makes most Court watchers' big three (or at least big five) list. But, in an unusual move, the opinion came down the first week of April.
The Court struck down aggregate limits on individual contributions to candidates for federal office, political parties, and political action committees. McCutcheon v. FEC will likely impact the dozen or so states that place aggregate limits on individual campaign contributions to candidates for state office.