Following a corporate fundraising scandal in Theodore Roosevelt's 1904 campaign, Congress passes the Tillman Act outlawing contributions by corporations in federal elections.

Congress tries to set contribution limits and disclosure requirements after the Teapot Dome Scandal, but the legislation leaves too many loopholes to be effective.

When Congress applies the Tillman Act to labor untions, they respond by creating PACs that can raise money and make campaign contributions as autonomous organizations.

When Congress passes the Federal Election Campaign Act, which it rewrites in 1974 following Watergate. It sets contribution limits and disclosure requirements and institutes public financing for presidential elections. As a result corporations are encouraged to increase their PAC activities.

The Supreme Court strikes down parts of the Federal Election Campaign Act (Buckley v Valeo) ruling that individuals may spend (but not contribute) unlimited money on political issues as an exercise in free speech. The ruling also establishes the principle that TV and radio ads that avoid express advocacy of a candidate are not subject to regulation, freeing independent groups to air issue ads attacking or promoting a candidate as long as they avoid hortatory language such as "elect" and "defeat."

Congress amends FECA to create the soft money loophole, allowing corporations, unions and the wealthy to give unlimited sums to national party committees for "party-building" activities.

Independent groups create political committees under the tax codes Section 527, but tell the Federal Election Commission they're not advocating for or against a specific candidate, thus skirting disclosure requirements. When Congress passes a tighter disclosure mandate in 2005, groups turn themselves into 501(c)4 advocacy organizations, again stretching the rules and further shielding themsevles from having to disclose contributors.

The McCain Feingold reform bill effectively closed down soft money activities, and also barrred "sham issue ads" produced by independent groups but financed by corporate or union money within 60 days of a general election.

With two new conservative justices, the Supreme Court reconsiders McCain Feingold's issue ad strictures. It ruls to permit union and corporate money for issue ads that don't contain the functional equivalent of express advocacy.

Adjudicating the controvertial Citizens United case, the Supreme Court decides corporations can, after all, advocate expressly for and against candidates, while leaving intact the 1907 proscription against direct corporate contirbutions in federal elections.