There is no reason to believe that the old system can be restored. Super PACs are here to stay, but they are not the only source of radical change. So-called s ocial welfare organizations — like the Koch brothers’ conservative Americans for Prosperity and similar organizations on the left — engage in just enough electoral activity to have a major impact, but not so much that they must comply with the same rules, such as disclosure regulations, that bind regular political committees.

Meanwhile, the storied 30- or 60-second TV ad is diminishing in importance. Digital politics consumes a steadily increasing share of campaign spending. The merest sliver of the population provides the private contributions on which campaigns run: In 2016, 2.76 percent of all individual contributors donated half of all individual money given to candidates, and just 1.31 percent of the voting-eligible population made any contributions at all.

When the campaign finance laws first came under stress, in the early to mid-1980s, reformers were convinced that the cracks could be repaired. Through the enactment of the McCain-Feingold Act, Congress endeavored to close the so-called soft-money loophole, and the Supreme Court upheld the core elements of this reform. But the new law ignited active opposition, expressed in ceaseless litigation and a divided Federal Election Commission. Then a majority of the Supreme Court shifted sharply toward a jurisprudence hostile to regulatory limits.

Many reform aspirations are defined by a wish to turn back the clock. But there is little appetite on either side of the polarized political divide for government to constrain the resources available to pursue ideological or partisan commitments. Activists skeptical of the parties will be unlikely to give up the alternatives offered by super PACs and “ social welfare” organizations. Similar obstacles lie in the path of extensive government regulation of online political speech and organizing.

In effect, where reform once aimed to have government structure campaign finance with comprehensive rules, we have something close to an unregulated political marketplace. There are benefits: innovation, as in the development of advanced technologies and new forms of organization. But there are also steep costs: Markets can fail, delivering well — especially for the affluent — on some but not all measures of democratic health and vibrancy, and opening up opportunities for new and dangerous sources of abuse such as the Russian electoral intervention of 2016. What remains of the old legal framework is largely a study in unintended consequences, such as the effect of restrictions on the political parties in weakening them and aiding the rise of the super PACs.