WASHINGTON — Federal judges grilled attorneys Monday over South Carolina’s controversial voter-ID law, which opponents say would disenfranchise thousands of minorities but supporters say includes ample protection against discrimination at the polls.

During closing arguments in a six-day federal trial over the law, the three-judge panel challenged attorneys for the state over election officials’ shifting stances on how they would implement the law.

But the judges also asked opposing attorneys why they are rejecting clear efforts by those election officials to soften the law’s possible harmful impact on African-American voters.

The S.C. law, which Attorney General Eric Holder blocked after its May 2011 enactment, has national implications, pitting a state’s right to prevent electoral fraud against the federal government’s mandate under the 1965 Voting Rights Act to ensure equal access to the polls for minority Americans.

The embattled law is one of more than a dozen that mainly Republican-majority state legislatures have passed in recent years. It would require a voter to show one of five forms of identification: a driver’s license, a photo ID issued by the state Department of Motor Vehicles, a passport, military ID or a voter registration card with a photo issued by the local elections office.

The law’s foes say it would affect black South Carolinians disproportionately because 71,000 registered African Americans lack any of the five IDs — a relatively bigger share than white voters who are without them. Opponents also contend those black voters would have more trouble obtaining acceptable photo IDs because they are poor and live predominantly in counties that have little or no public transportation.

Garrard Beeney, an attorney for civil rights groups and potentially disenfranchised voters who oppose the law, ridiculed testimony by Marci Andino, the executive director of the State Election Commission, as contradictory. He said the state’s thousands of precinct volunteers would be left to interpret whether someone could vote without proper ID by claiming a “reasonable impediment” to getting one, as permitted by one of the law’s key clauses.

U.S. District Judge John Bates pointed out S.C. officials had promised to distribute instructions for implementing the law. Beeney responded the law is irreparably flawed: Poll volunteers, he said, are being asked to implement it in ways that it doesn’t allow or that other laws prohibit.

Bates asked: “Are there cases where a court has said it doesn’t trust the way a state says it’s going to interpret the law?”

Beeney replied that trust isn’t the issue because even with the best intentions, state election officials lack legal authority over county election boards.

“There is no doubt and no dispute that the county election commissions are the ones that control elections,” Beeney said.

He urged the judges to reject the state’s assurances that “reasonable impediment” affidavits, which voters without the required IDs must sign explaining why they didn’t obtain one, would be accepted without being notarized despite legal requirements that notaries witness affidavits.

Judge Brett Kavanaugh, a U.S. Appeals Court judge who is sitting on the federal district bench panel for this trial, chided Justice Department lawyer Matthew Colangelo for rejecting pledges by S.C. Attorney General Alan Wilson, Andino and other state officials that they will interpret the law leniently and err on the side of the voter.

“There was ambiguity when we started,” Kavanaugh told Colangelo. “But we’ve come a long way, and we’ve come a long way in your direction.”

Kavanaugh and Colangelo engaged in spirited banter over whether state officials’ interpretation of the law can trump the law’s provisions when they are contradictory.

“South Carolina has been proposing a very expansive interpretation of the ‘reasonable impediment’ provision, yet DOJ rejects that very expansive interpretation,” Kavanaugh said. “They’ve interpreted the law to say a notary is not required (for the affidavits). Accept that! Why is that not a victory? But you’re fighting it.”

Colangelo responded state officials can’t ignore the law’s requirements or legislators’ intent in passing it as they enforce it.

Bates indicated if the court upholds the law, it might prohibit the measure from being used in November’s elections because there’s too little time to educate voters and train poll workers.

In a separate development, U.S. Sen. Lindsey Graham and U.S. Rep. Trey Gowdy, both South Carolina Republicans, joined House Judiciary Committee Chairman Lamar Smith, a Texas Republican, in writing a letter to Holder in which they claimed the attorney general had overruled his staff’s recommendations that the Justice Department approve the S.C. law.

“It is troubling that scarce funds are likely being wasted opposing common-sense legislation that was cleared by the department’s own nonpartisan experts,” the lawmakers wrote.

A separate three-judge panel last month upheld Holder’s rejection of a similar Texas voter-ID law, saying it violates the Voting Rights Act. The U.S. Supreme Court is expected to rule on the South Carolina and Texas laws, possibly in tandem as a single decision.