It was in a rare burst of bipartisanship that Congress in 2010 passed the Fair Sentencing Act, which attempted to close the sentencing gap between those convicted of crack cocaine offenses, who tend to be black, and those with powdered cocaine offenses, who tend to be white.

But Congress left a key bit of the legislation unexplained: whether the reduced sentences for crack should apply to those who committed their crimes before the law took effect but who were not sentenced until afterward.

An hour-long argument at the Supreme Court on Tuesday did not seem to produce a definitive answer.

Lawmakers intended for the new sentencing structure to take effect immediately, said Stephen B. Eberhardt, a lawyer arguing for two Illinois men who did not receive the benefits of the measure even though they were sentenced after President Obama signed it in August 2010.

“Why would Congress want district courts to continue to impose sentences that were universally viewed as unfair and racially discriminatory?” he asked.

But lower courts have disagreed on the question, citing an 1871 law that says when Congress means for a change to apply retroactively, it must “expressly provide” for that.

In the case of Eberhardt’s clients, Corey A. Hill and Edward Dorsey Sr., the U.S. Court of Appeals for the 7th Circuit said the 1871 measure prevented it from reading the sentencing act as the men would like. The panel of judges in Dorsey’s case called the new law the “Not Quite as Fair as It Could Be Sentencing Act of 2010.”

Chief Justice John G. Roberts Jr. told Eberhardt that the court must operate under the “proposition that Congress, when it enacts legislation, knows the law.”

And complicating the issue for the court is a reversal from the Obama administration. When the law was passed, the administration’s prosecutors told judges that the reduced sentences should apply only to those who committed their offenses after the legislation was enacted.

But after Democratic lawmakers complained, Attorney General Eric H. Holder Jr. changed the policy and said the new structure should be applied to those who had not yet been sentenced, regardless of when their offenses occurred. It is unclear how many defendants fall into that category.

Deputy Solicitor General Michael R. Dreeben told the justices that sentences should be based on the law at the time of sentencing. He said it is clear that Congress would want such a rule followed in these cases, because the racial inequalities that came with the disparate sentences for crack vs. powder cocaine “had undermined the credibility of the criminal justice system for years.”

Justice Antonin Scalia, the justice who seemed most opposed to the administration’s reading of the law, responded: “Yes, that’s very nice, but let’s talk about text, not . . . about the emotions of Congress.”

The Fair Sentencing Act changed a regime that essentially treated one gram of crack cocaine as the equivalent of 100 grams of powder cocaine. It reduced that ratio to about 1 to 18 and thus raised the possession amount of crack cocaine necessary to trigger mandatory sentences. About 85 percent of those convicted of possessing crack cocaine are black.

Because the Obama administration now disagrees with the sentences federal prosecutors sought for Dorsey and Hill, the court appointed Washington lawyer Miguel A. Estrada to defend the 7th Circuit decision.

He said that sentencing law is supposed to treat similarly situated people alike. “People who committed the same offense on the same date and may have done so with each other, we would expect to get comparable punishment,” Estrada said.

But Justice Sonia Sotomayor said circumstances were different when Congress found its laws discriminatory.

“Once Congress has said, ‘This law’s not being enforced in a race-neutral way, we want to fix it,’ why shouldn’t our presumption be that the fix is immediate rather than delayed?” she asked Estrada.

When he responded that it would be “wrong to assume that the passage of the act reflects Congress’s concession of intentional discrimination,” Sotomayor was not satisfied.

“Mr. Estrada, I’ve been a judge for nearly 20 years, and I don’t know that there’s one law that has created more controversy or more discussion about its racial impact than this one,” she said.

The cases are Dorsey v. United States and Hill v. United States .