But even supporters of the rules acknowledge that the benefits can be hard to measure. “I happen to think the campaign finance laws have done some modest good,” said Richard L. Hasen, an expert on political law at the Loyola Law School in Los Angeles. “How much good? We may soon find out,” he added, in the aftermath of the Supreme Court’s ruling on Thursday in Citizens United v. the Federal Election Commission.

Supporters of the restrictions point to Britain to show that governments can police corruption without imperiling free speech. Britain started regulating political spending as far back as 1883 and has tightened the rules steadily ever since.

Those British restrictions would violate the Supreme Court’s view of the First Amendment, yet Britain’s political debates are as robust as they are in the United States.

Opponents of restrictions, on the other hand, point out that Australia barely regulates political money. Individuals and corporations can give without limit. Parties can spend freely. And there is not much disclosure about who gives what to whom. But political corruption has not threatened a vibrant democracy there.

In the United States, studies comparing states like Virginia with scant regulation against those like Wisconsin with strict rules have not found much difference in levels of corruption or public trust, several scholars said. Jeff Milyo, an economist at the University of Missouri, has compared states with strict bans on corporate contributions to political parties against those with no limits at all. “There is just no good evidence that campaign finance laws have any effect on actual corruption,” he said.

The most insistent advocates of the campaign finance laws argue that the benefits are real even if academics can’t measure them. Fred Wertheimer, the dean of campaign finance “reformers,” pointed to the presidential campaign finance system as the best example of success. For five elections beginning in 1976, the presidential candidates of both major parties took public financing and did not receive private campaign contributions. “You can’t prove a negative,” Mr. Wertheimer said, “but in the Carter and Reagan presidencies there were no news stories about campaign contributions influencing presidential decisions.”

By the 2008 election, however, that system had grown obsolete. Candidates could raise far more from private donors, and President Obama became the first major candidate since Richard M. Nixon to win election without public money.