WASHINGTON — If you’re trying to cure an illness, and you get better, but not entirely — say you had a high fever, but now you have the sniffles and a sore throat — does it make sense to keep taking the same medicine? What if your doctor insists?

Justice Stephen Breyer offered the disease analogy Wednesday morning for racist efforts to block the power of black and Hispanic voters in the South during a sharply polarized argument — 5 to 4, conservatives v. liberals — over whether Shelby County, Ala., has taken enough medicine from Section 5 of the Voting Rights Act. Congress first enacted the Voting Rights Act in 1965 to deal with massive and violent suppression of black voters in the South. The problem was so entrenched that when federal courts would strike down a discriminatory measure like a poll tax, Southern states and counties would quickly dance around the ruling, enacting new barriers such as a literacy test. So Congress armed the Voting Rights Act in two ways. The first, Section 2, bans any voting practice that discriminates on the basis of race or ethnicity. It applies uniformly, throughout the country, and it has no expiration date. To enforce it, the government, or a group or person affected by the law, has to sue — and has the burden of proof. The second part of the Voting Rights Act, Section 5, relied on data showing a pattern of discrimination at the time to create a category of “covered jurisdictions.” Congress said that for 25 years the Department of Justice had to “pre-clear” any changes to voting rules in those places, or else the state or county had to go to court for approval before the changes could go into effect. The list of covered jurisdictions included most of the South, along with a smattering of counties and cities in other states.

The list of jurisdictions where Section 5 applied remained the same when Congress last reauthorized the Voting Rights Act in 2006. But some of those states say they no longer belong — they’re cured, and that means it’s time for the whole Section 5 formula to go. That’s the question the court was grappling with on Wednesday: In 2006, given the history and the current evidence, did Congress have enough reason to think that the South still suffered from Racist Voting Disease? What’s the test — does every single state, city and county covered by Section 5 have to be worse than every single state, city and county in the rest of the country? And what kinds of symptoms qualify for Section 5 medicine — low turnout or registration rates, proof of the old forms of blatant discrimination, or more subtle measures, like moving a polling place out of a minority neighborhood?

Shelby County’s lawyer, Bert W. Rein, says Section 5 is over and done. Quoting the court’s last ruling on the matter from 2009, he opened by saying that the record before Congress the last time around “made it unmistakable that the South had changed.” Justice Sonia Sotomayor was the first to pounce. She said she accepted that some portions of the South had changed, “but your county hasn’t.” She cited 240 discriminatory voting measures recently blocked by Section 5 and Section 2 challenges. “You may be the wrong party bringing this suit,” Sotomayor continued, calling Shelby County the “epitome” of the reason for keeping Section 5 in place.