“THIS IS NOT the kind of a question you can leave to Congress,” Justice Antonin Scalia pronounced during a Supreme Court argument Wednesday.

The subject was the Voting Rights Act, one of the most successful pieces of civil rights legislation in U.S. history, and in particular its Section 5. That provision requires nine states and assorted jurisdictions in seven others to win Justice Department approval before changing their voting laws. The burden is on these “covered” jurisdictions, unlike the rest of the country, to prove that such changes won’t adversely affect the right of African Americans and other minorities to vote. Shelby County, Ala., was in court Wednesday arguing that this pre-clearance requirement is an unfair infringement on Alabama’s sovereignty.

Congress adopted the Voting Rights Act in 1965 and has reauthorized it four times since, most recently in 2006. Each time it has extended Section 5 on the grounds that some jurisdictions, mostly but not only in the South, have particularly egregious records of interfering with blacks voting. Originally such interference took the form of “literacy tests” and other blatant blocks to voter registration; these days it can involve more subtle ways of limiting minority turnout, such as moving polling stations. The reauthorization in 2006 was based on a voluminous record — thousands of pages of evidence — showing that the covered jurisdictions have a propensity toward discriminatory rules. The Senate agreed by a vote of 98 to 0; the House, 390 to 33.

“It was clear to 98 senators, including every senator from a covered state, who decided that there was a continuing need for this piece of legislation,” Justice Elena Kagan said, in what might seem a self-evident point.

But not to Justice Scalia. “Or decided that perhaps they’d better not vote against, that there’s . . .none of their interests in voting against it,” he said. Later he elaborated on why he feels free to dismiss this particular congressional action: “I don’t think there is anything to be gained by any senator to vote against continuation of this act. . . . They are going to lose votes if they do not reenact the Voting Rights Act. Even the name of it is wonderful: the Voting Rights Act. Who is going to vote against that in the future?”

This is a stunning line of argumentation. Congress is empowered to write legislation enforcing the Fourteenth and Fifteenth Amendments. But if Justice Scalia doubts the purity of lawmakers’ motives, then apparently this power is limited. We wonder how the justice is able to discern what lay within the hearts of these 98 senators. We also wonder how many challenged acts of Congress would survive if the court saw fit to strike down any that were enacted by lawmakers considering, in part, their reelection prospects.

Chief Justice John G. Roberts Jr., who expressed skepticism about the Voting Rights Act on different grounds, has been eloquent on the court’s responsibility to defer to Congress and the democratic process when appropriate. We hope he will not want to associate himself with Justice Scalia’s contempt. Congress, after careful review, came to an overwhelming conclusion that protection of the franchise in America is much improved but not guaranteed, especially in certain areas. We heard in Wednesday’s argument no grounds for the court to claim superior wisdom on that question.