Mr. Obama was wrong in almost every respect about Citizens United v. Federal Election Commission, which the court decided on Jan. 21, 2010. Hysterical predictions about Citizens United—then-Rep. Ed Markey, among others, compared it to Dred Scott—haven’t held up.

Contrary to Mr. Obama’s assertion about a century of law, Citizens United overturned portions of McCain-Feingold, a campaign-finance law that wasn’t even 10 years old, and another law from 1947. Those laws prohibited unions and corporations, including nonprofits, from voicing support for or opposition to candidates for federal office.

Citizens United didn’t affect the longstanding ban on corporate contributions to candidates, and it didn’t legalize foreign political spending in the U.S. Most Russian online ads in 2016 would have been protected under the First Amendment even before Citizens United, because the ads didn’t urge a vote for or against a candidate.

Far from handing power to the 1%, Citizens United unleashed rapid political diversification. Since the ruling, the White House or Congress has changed parties in every federal election except 2012. Twenty eighteen saw the highest midterm voter turnout in a century. Small-dollar donors are more coveted than ever. Donald Trump raised more money from donors who gave less than $200 than any candidate in history.

Since Citizens United, party outsiders such as Mr. Trump and Bernie Sanders have risen to national prominence. And money hasn’t been able to buy elections as predicted. Sheldon Adelson donated record amounts to Republican super PACs in 2012 but failed to prevent strong Democratic victories. Democrats Tom Steyer and Michael Bloomberg came up empty after putting huge sums of money behind climate change and gun control.

Hillary Clinton outspent Mr. Trump 3 to 1 in 2016. Congressional leaders and big-time fundraisers such as Reps. Eric Cantor (R., Va.) and Joe Crowley (D., N.Y.) lost their seats to primary challengers who spent a fraction of what the incumbents did. Incumbent re-election rates in the House never dipped below 94% from 1996 to 2008, but did in 2010, 2012 and 2018.

Citizens United deserves a share of credit for all these trends. The decision made it easier to promote (or criticize) a candidate without help from party leaders or media elites.

Perhaps the worst prediction was that Citizens United would allow a corporate takeover of democracy. The New York Times accused the justices of having “paved the way for corporations to use their vast treasuries to overwhelm elections” and “thrust politics back to the robber-baron era of the 19th century.”

A decade later, most spending comes from the same place it always has: individuals who donate directly to candidates, up to legally limited amounts. Corporations contribute well under 10% of federal political spending, Their voice is not dominant—and voters have a right to hear it. Justice Anthony Kennedy and his colleagues didn’t hold that “money is speech” or “corporations are people.” The ruling was part of a healthy shift in favor of free speech in politics—a trend that began with 2007’s Wisconsin Right to Life v. FEC, and continued through 2014’s McCutcheon v. FEC.

The questions is whether the justices think their work is done. If they truly want to empower democracy, they should continue to look skeptically at regulation of campaign finance. Political speech, after all, is at the core of the First Amendment’s protection.