Mr. McCutcheon contends that striking down the overall limit would not undermine protections against corruption because an individual would still be bound by the per-candidate limits. But this ignores the reality of modern campaign finance, in which politicians and party committees solicit large sums from individual donors and then funnel the money through fund-raising committees to particular candidates. Without the current cap, the government argues, a single donor could give up to $3.6 million to a party’s political candidates and committees in an election cycle.

The very wealthiest Americans already have disproportionate influence: in the 2012 election, 1,219 donors reached or nearly reached the overall limit, and together they were responsible for giving $155 million to federal races.

One study, by Demos and the United States Public Interest Research Group, projected that without the overall limit in place, those donors would have contributed nearly triple that amount — or nearly 50 percent more than President Obama and Mitt Romney received from all small donors combined. This is significant because a recent report by political science researchers showed that the wealthy differ significantly from ordinary Americans in their policy preferences. For example, job creation is consistently the top priority for most Americans, but the budget deficit is the most pressing problem among the wealthiest.

Sensible contribution limits do not limit speech. As the court recognized in the Buckley case, they are a “quite modest restraint” that help prevent evasion of the per-candidate limit. An individual like Mr. McCutcheon may still say whatever he wants, and he may spend all he would like on independent campaign-related messages.

The nation’s founders understood the threat of corruption in politics and were preoccupied with combating it. The government should be dependent on “the great body of the people,” James Madison wrote, and not “an inconsiderable proportion, or a favored class of it.” If the court is going to help protect American politics from becoming little more than “a disagreement among rich people,” as one observer put it, it should follow its own precedent and uphold the overall contribution limit.