In the winter of 2000, just as John McCain and George W. Bush were entering a pivotal series of Republican primaries, a mysterious new campaign ad appeared on television stations in New York, Ohio and California. The ad showed McCain’s disembodied head floating over belching smokestacks as a narrator intoned about votes McCain had made against solar and renewable-energy incentives — policies that in reality he had supported. The ad was sponsored by a group called Republicans for Clean Air.

The financial backers behind Republicans for Clean Air were the Texas billionaires Charles and Sam Wyly. They had a history of supporting Bush, but they were also heavily invested in a company seeking government-induced increases in alternative-energy production. The revelation that a single Texas family could so easily insert itself into the political process set off a round of frenzied indignation, and after McCain’s loss, it became crucial fodder for his effort to reform the national campaign-finance system. In 2002, along with a Democratic senator, Russell D. Feingold, he helped push a bill through Congress that ranked alongside some of the most sweeping efforts to contain “special interest” money in American history: the Tillman Act of 1907, which banned corporate contributions to candidates; the Smith-Connally Act of 1943, which prohibited union donations to candidates; and the Federal Election Campaign Act of 1971, which placed strict limits on what individuals could give to parties and campaigns.

Before 2002, parties could accept unlimited donations from individuals or groups (corporations, labor unions, etc.) so long as they devoted the funds — so-called “soft money” — to the amorphous act of “party building.” The McCain-Feingold law, as it came to be known, banned soft-money contributions, and it also prohibited political groups that operate outside the regulated system and its donation limits — like the Wylys and their Republicans for Clean Air — from running “issue ads” that appear to help or hurt a candidate close to an election. It implemented tough fines and even prison terms for those who illegally coordinated with the official campaigns.

In 2010, the Citizens United decision by the Supreme Court effectively blew apart the McCain-Feingold restrictions on outside groups and their use of corporate and labor money in elections. That same year, a related ruling from a lower court made it easier for wealthy individuals to finance those groups to the bottom of their bank accounts if they so chose. What followed has been the most unbridled spending in elections since before Watergate. In 2000, outside groups spent $52 million on campaigns, according to the Center for Responsive Politics. By 2012, that number had increased to $1 billion.

The result was a massive power shift, from the party bosses to the rich individuals who ran the super PACs (as most of these new organizations came to be called). Almost overnight, traditional party functions — running TV commercials, setting up field operations, maintaining voter databases, even recruiting candidates — were being supplanted by outside groups.