If you hate the current state of campaign finance, in which corporations and non-profits exert influence through trade associations, political action committees and so-called super PACs, you can’t lay all of the blame at the doorstep of the U.S. Supreme Court’s 2010 ruling in Citizens United v. Federal Election Commission, which held that corporations and labor unions have the same First Amendment rights to free speech as individuals.

Nor can you say that the root of the problem was the court’s 2007 ruling in Federal Election Commission v. Wisconsin Right to Life that corporations and labor unions are permitted to spend money on election ads as long as those ads do not contain “express advocacy” for or against a candidate.

Instead, you have to look back to 1976, when the Supreme Court decided in Buckley v. Valeo that the constitution permits limits on direct campaign contributions to candidates by corporations. Such restrictions, the Buckley court held, do not violate the First Amendment.

That bar on direct contributions to candidates, reaffirmed by the U.S. Supreme Court in 2003 in FEC v. Beaumont, has remained in place despite repeated assaults in recent years. As Rick Hasen, an election-law expert at the University of California, Irvine, School of Law wrote Wednesday at his Election Law Blog, the current justices may well overturn Beaumont‘s holding on direct corporate contributions to candidates if they decide to take up the issue, but so far they haven’t.

Direct contributions, however, weren’t the only issue in that seminal 1976 ruling in Buckley. The court drew a line between direct contributions and “independent expenditures,” in which people (or groups) exercise their First Amendment rights to express political support for a candidate. Buckley said that while corporations can’t make direct contributions, people and groups cannot be barred from independent political expenditures. More than a quarter-century later, in Citizens United, the Supreme Court explicitly extended that reasoning to corporations and unions, ruling that the First Amendment protects their right to independent political expenditures.