On Friday morning, Judge Robert L. Wilkins looked out across the packed courtroom at the lawyer for Texas and suggested that the state’s voter ID law would force some people to travel more than 100 miles to get the documents required for a photo identification.

“How does that impact your argument?” asked Wilkins. “Isn’t that unduly burdensome?”

John Hughes, the state’s attorney, said Texans in rural areas are used to driving long distances. “People who want to vote already have an ID or can easily get it,” he said.

The exchange highlighted the key dispute in a historic case that played out in final arguments Friday before a three-judge panel in U.S. District Court in Washington. The issue is whether the 2011 law violates the federal Voting Rights Act by making it harder for minorities to cast ballots.

The stakes go far beyond Texas. For the Obama administration, the Texas law and similar legislation passed in other states threaten to disenfranchise millions of Latino and African American voters. But for supporters of the legislation, the requirements are common-sense solutions to voter fraud.

The jammed courtroom underscored the magnitude of the case. Among the spectators were attorney Stephen Pollak, who helped write the 1965 Voting Rights Act, which was designed to protect minority voters, and Thomas Perez, the head of the Justice Department’s civil rights division. Pollack also defended the law before the Supreme Court.

Voting Rights Act cases must be decided by a panel of three federal judges. Any appeal — and either side is expected to appeal an adverse ruling — will go straight to the Supreme Court, bypassing an appeals court.

Texas has a history of voter discrimination, so the law signed last year by its Republican governor, Rick Perry, had to be cleared by the Justice Department. The department blocked the law, saying it would disproportionately affect Latino voters. Texas sued the department, leading to the week-long trial that ended Friday.

Much of the debate over the law has focused on the issue of exactly who would be affected by it. The burden is on Texas to convince the panel that the voter ID law is not discriminatory, and on Friday, Hughes, the state’s attorney, argued that there is no evidence, statistical or otherwise, that it is.

That argument, however, was received skeptically by one of the jurists, David S. Tatel, of the U.S. Court of Appeals for the District of Columbia.

“The record tells us there is a subset of registered voters who lack ID,” said Tatel, who was appointed in 1994 by President Bill Clinton. “We have to think about the economic burden and the fact that minorities are disproportionately poor.”

Tatel added that the record showed that minorities in Texas are more likely than whites not to have cars and to live up to 120 miles away from the closest place to get voter ID documents.

Under the Texas law, the minimum cost to obtain a voter ID for a state resident without a copy of his birth certificate would be $22, according to the Justice Department. While the “election identification certificate” needed to vote is free, the state legislature voted down a proposal to allow people to get the documents needed for the voter ID for free.

In his closing argument, Justice lawyer Matthew Colangelo said that the Texas law will disenfranchise more than a million African American and Hispanic voters and “is exactly the type of law” that Congress had in mind when it passed the Voting Rights Act.

Republican lawmakers have argued that the voter ID law is needed to clean up voter rolls, which they say are filled with the names of illegal immigrants, ineligible felons and the deceased. Texas, they argue, is asking for no more identification than people need to board an airplane, get a library card or enter many government buildings.

During the trial, the judges heard testimony from lawmakers, professors, civil rights activists, election lawyers and statisticians.

But some of the expert testimony, especially arguments over the estimated number of people who could be adversely affected, was inconclusive.

On Thursday, Harvard University political science professor Stephen Ansolabehere, an elections and statistics expert, testified for the Justice Department that the Texas law was more likely to affect Hispanic and black voters than white ones.

But Texas lawyers countered that Ansolabehere’s research methods were “hopelessly flawed,” and said a list he produced of 1.5 million potential Texas voters without state-issued IDs was inaccurate.

To buttress their argument, the lawyers showed that Ansolabehere’s list included Sen. Kay Bailey Hutchison (R), former president George W. Bush and former senator Phil Gramm (R).

Ansolabehere said his analysis was imperfect, in part because he was not given access to federal or military data to cross-check his list against the Texas voter registration list.

The trial unfolded against the backdrop of a fierce national debate over voting rights during an election year. Earlier this week, Attorney General Eric H. Holder Jr. vowed to challenge voting laws such as the one in Texas, calling them “poll taxes” because of the costs associated with getting voter IDs.

The remark, a reference to fees that were imposed during the Jim Crow era, angered Republicans, who accused the attorney general of playing “identity politics.”

The three judges, including Rosemary Collyer, appointed in 2002 by Bush, have said they will decide the case by Labor Day.

Julie Tate, Annelise Russell and Lindsey Ruta contributed to this report.