There is no authoritative estimate of the amount of soft money raised during this period, but for the 10 years from 1992 (when regulators began to keep track) to 2002, soft money donations totaled $1.53 billion.

The effective life span of the 2002 McCain-Feingold Act was even shorter. By 2004, the first election governed by McCain-Feingold, organizations claiming to be exempt from B.C.R.A. prohibitions on million-dollar-plus contributions managed to funnel $478.2 million in soft money into the election anyway. Some of the major recipients were the pro-Democratic Americans Coming Together, the pro-Republican Swift Boat Veterans for Truth and pro-Republican Progress for America.

Since 2002, there has been a steady erosion of the power of the political parties, as opposed to that of independent expenditure committees. While such committees cannot coordinate spending with candidates or parties, they can independently support candidates, and in doing so they have filled much of the vacuum created by the restrictions of money raised and spent by the parties.

Many voters, told that the power of the major political parties was diminishing, would say that they were happy with this development, but the consequences are not quite what they hoped for. The major Republican and Democratic committees — the Republican and Democratic National, Congressional and Senatorial committees — remain restricted by law in the size and source of contributions they accept, while independent expenditure committees, particularly super PACs, are now subject to virtually no restraints. McCain-Feingold “was a major blow to political parties, which are still struggling to survive,” Jan Baran, a Republican campaign finance lawyer, wrote in an email to me:

The outlawing of soft money not only eliminated needed funding, it predictably enhanced the role of large existing outside groups (NRA, Sierra Club, etc.) and immediately spawned the creation of new groups (527 organizations, tax exempt social welfare organizations, etc.).

”Subsequent court decisions,” Baran added, further exacerbated the “imbalance between the roles of independent groups and the parties.”

As campaign finance law has been repeatedly roiled, who has gained and who has lost?

At every stage, business interests have steadily grown in power. Watergate-era reforms, for example, gave legal sanction to the creation of corporate and union political-action committees — a type of committee that had been primarily the province of trade unions before the 1974 legislation was enacted. The result was a surge in the number of corporate PACs from 89 in 1974 to 1,677 to July 2015, while the number of union PACs grew modestly, from 201 to 277.