Congress would love to have complete discretion to regulate anything related to political campaigns. That would make it easy for lawmakers to address (or at least claim to address) public concerns about potential corruption, while also giving incumbents the means to assure that they remain incumbents.

But the convergence of debate and voting implicate something very important: the First Amendment, which was intended to protect freedom of speech, particularly speech about political matters. As the Supreme Court reminded elected officials Monday, that constitutional mandate puts some types of regulation firmly off-limits.

The decision involved ads that Wisconsin Right to Life wanted to broadcast during the 2004 election campaign. The spots denounced efforts to block Senate votes on President Bush's judicial nominees and urged citizens to contact their senators, Herb Kohl and Russ Feingold. But the 2002 federal campaign finance law made it a crime for corporations or unions to run ads mentioning a candidate for office (which Feingold was) within 30 days of a primary or 60 days of a general election. So Wisconsin Right to Life went to court arguing that the ban infringed on its freedom of speech. This week, by a 5-4 vote, the Supreme Court agreed.

Previously, the court had said Congress may ban such ads if they amount to attempts to influence the outcome of an election — even if the commercials don't specifically advocate voting for or against a candidate. But as Chief Justice John Roberts explained in the court's opinion, that doesn't give the Federal Election Commission the power to suppress genuine "issue ads" merely because they mention a candidate.

As a rule, he said, only spots that are the "functional equivalent" of campaign ads may be banned. If, on the other hand, a commercial "may reasonably be interpreted as something other than an appeal to vote for or against a specific candidate," it must be allowed.

The reason for this distinction is clear. It's a healthy thing for citizens or corporations and unions to express their views, even — or perhaps especially — during election season. Barring ads like the Wisconsin group's would stifle debate, depriving the public of potentially valuable information.

Dissenting justices David Souter, John Paul Stevens, Ruth Bader Ginsburg and Stephen Breyer railed against the decision, denouncing the high cost of modern campaigns and "the electoral leverage of concentrations of money." These laments suggest they think their role is to advance what they see as wise policy. In fact, the job of the court is to assure that laws, however well-intended, comply with the demands of the Constitution.

True, the majority said, the federal government has an interest in combating corruption, real or imagined, which is why it may limit contributions to candidates. But concerns about graft, it said, can justify regulation of issue ads only when they are merely campaign commercials in disguise. It's not too much to ask that the FEC show that an issue ad is phony before it prohibits it.

Regulation of campaigns is a legitimate function of government, but only if it respects the basic freedom of Americans to debate the issues before them. In this case, wrote the chief justice, "we give the benefit of doubt to speech, not censorship." That's exactly where it belongs.