WASHINGTON — Ten years ago, an earthquake rumbled through this city and then beyond the Capital Beltway, sending shock waves into presidential politics and even into the state legislatures. It changed the way candidates campaigned and almost certainly altered who won those campaigns. It spawned an important debate about the role of money in politics and perhaps an even more significant debate about the nature of free expression in a free society.

Few Supreme Court decisions have had the immediate effect on the structure of American civic life as Citizens United v. FEC, which set in motion substantial changes in how campaigns are financed, disrupted the power of the established parties, boosted the influence of individual donors making independent contributions and — surprise! — increased the power of labor unions at the expense of major corporations.

“Politics was always a money game,” said former Labor Secretary Robert B. Reich, who now teaches at the University of California, Berkeley. “But after Citizens United, money had the potential of being a flood, and getting rid of it became far harder to do.”

In its wake, Citizens United has transformed individuals such as Sheldon Adelson of Las Vegas Sands (who made $122.3 million in contributions during the 2018 midterm congressional elections) and two current presidential candidates — former New York Mayor Michael Bloomberg, who made $95 million in 2018 contributions, and entrepreneur Tom Steyer, who made $72.8 million in contributions — into the new power brokers of American politics.

But it isn’t only the big funders. Overall, individuals contributed more than $294 million to campaigns in 2014, four years after the Citizens United decision. That amount jumped to $1.1 billion two years later, representing two out of every three dollars in the election that year.

The ruling has undermined the power and influence of party campaign committees, as recently as a decade ago major providers of campaign funding and, just as significant, important sources of influence on the content of House and Senate races and the selection of candidates for those positions. With independent contributions by individuals far surpassing the amounts provided by the two parties, candidates no longer are subject to the discipline of party leaders, leading to far more populist candidates in House races and the diminution of party discipline.

“The post-Citizens United regulatory environment has clearly led to a change in the sources of campaign money, with a substantial share of funding now coming from entities that are not subject to contribution limits,” according to a report by the Committee for Economic Development of the business-oriented Conference Board. “This is primarily due to the rise of Super PACs, which have become the primary recipients of unlimited contributions.”

The flood of big-corporation contributions into presidential and congressional campaigns never materialized. Not one major corporation made an independent expenditure in the past two election cycles. Publicly held corporations accounted for less than 1% of total Super PAC contributions in 2016. Trade associations have been even less prominent, also accounting for less than 1% in 2016, with almost all of it coming from one group, the National Association of Realtors.

One study found that the decision, and the changes in campaign financing it prompted, bolstered the prospects of Republican incumbents in state-legislative contests and discouraged potential Democratic candidates from entering legislative contests. “In the long run,” according to the study published in the Journal of Law and Economics, “this change has the potential to influence a wide range of policies.”

With all the data that has flowed about the sources of big-money individual contributions, the donations made by nonprofits are more opaque than ever — and perhaps more influential than ever. “The black hole of Citizens United has been the fact that nonprofit organizations now can make independent contributions without making disclosures,” said Anthony Corrado, a Colby College political theorist who is regarded as a leading expert in campaign finance. “Since the ruling, there’s been a dramatic decline in the nonprofit groups that disclose their donors.”

Criticism of Citizens United has only grown since the ruling was handed down a decade ago.

“This is not improving the democratic process but instead is weakening it,” says Steve Westly, a former controller and chief fiscal officer of California who was the state’s co-chair for Barack Obama and now is raising money for the presidential campaign of former Vice President Joseph R. Biden Jr. “It is enabling small groups of the very wealthy of the right and left to have undue influence over politics.”

Former Rep. James Leach, for 20 years a Republican member of the House who now teaches at the University of Iowa, argues that the decision “undermines the functioning of democracy.” Former Gov. Michael S. Dukakis of Massachusetts, the 1988 Democratic presidential nominee, adds: “After all the battles we’ve had over campaign finance, the court comes along and says what the Constitution did not say, which is that money is speech.”

That has prompted one of the great debates in American history — and a fresh movement for a constitutional amendment to overturn the Citizens United decision.

That proposed amendment includes a statement that its goal is to “advance democratic self-government and political equality, and to protect the integrity of government and the electoral process.” It would permit Congress and the states to regulate and set “reasonable limits on the raising and spending of money by candidates and others to influence elections.”

The effort has the support of the historian Doris Kearns Goodwin and former Republican Sen. Olympia Snowe of Maine.

“There is no question about the trend and changes that have occurred with money in general, no matter whether it comes from billionaires or corporations or labor unions,” said Jeff Clements, president of American Promise, a bipartisan group heading the effort to pass the amendment. “The money isn’t coming from ordinary Americans.”

On that there is no disagreement.

“The rise in the number of individual donors and the concentration of giving among a select group of them,” according to a Bipartisan Policy Center study by scholars affiliated with the New York University and Stanford law schools, “reflects a broader trend in the campaign finance system toward participation by individuals and ideological organizations chiefly funded by individuals, as opposed to other types or economic interests.”

That is the new reality, created by the Supreme Court — but mastered by individual donors who increasingly are displacing the power of the ancient political parties.