The US department of justice has defended its ban of a Texas voter ID law by telling a federal court that the legislation is an attempt by the state's largely white Republican party to resist the political impact of changing racial demographics.

A five-day hearing in Washington over whether the Obama administration has the power to block Texas from requiring voters to produce photo identification wrapped up on Friday with justice department lawyers strongly challenging the motives behind the legislation.

Texas says it is intended to combat a rising tide of voter fraud by requiring people to show specified forms of identification, such as a driving or gun licence, at the ballot box.

The justice department said the law was racially motivated because it was constructed by the Republican-controlled Texas legislature to disadvantage the growing Latino and black communities in the state which heavily favour the Democratic party.

The outcome of the case, expected in August, is likely to have an impact in other states where the justice department is challenging similar laws, including South Carolina, Mississippi, and Florida.

The issue is expected to end up before the supreme court.

An administration lawyer, Matthew Colangelo, told the court on Friday that as many as one in ten voters in Texas could be disenfranchised under the law because of the difficulties poor people face in getting the necessary identification, such as having to travel long distances or pay for copies of birth certificates. He said that was the intent of the law.

"Four million new people moved into Texas between 2000 and 2010. Ninety percent of those were minorities. Texas acted to take away that voting strength on the brink of that minority exercising it," he said. "It was enacted against the backdrop of a huge explosive growth in the Hispanic population."

The administration also contends that the costs involved in obtaining identification amount to a poll tax forbidden by the constitution.

A member of the Texas legislature, Trey Martinez Fischer, told the court that the voter ID bill was rushed through without proper debate using measures such as declaring it emergency legislation.

"There was a determined effort to pass this bill in record time," said Martinez, who chairs the Mexican American Legislative Caucus.

Another Democratic legislator, Wendy Davis, said her attempts to introduce amendments to the legislation that would help voters obtain identification, such as making it available at every county courthouse so that voters did not have to make roundtrip journeys of up to 250 miles to special offices, were blocked by Republicans.

"The voter ID bill, we all believed, was a racially motivated bill," she said.

Henry Flores, dean of St Mary's University graduate school in San Antonio, said in evidence that the voter ID law was only the latest of scores of pieces of legislation in response to significant demographic changes over the past 40 years.

"It seemed to me that [the ID law] had been turned into an anti-immigration bill," he told the court. "Race was at the heart of the whole thing."

A California Institute of Technology professor, J. Morgan Kousser, told the court that Texas Republicans had been trying to push through a voter ID law since 2005 and that politics in the state is "very racially polarised", with the state's Democratic party now dominated by Latinos.

"In 2011, the vast majority of Democratic legislators are minorities. So a bill that has partisan effects would have racial effects," he said.

The Obama administration blocked the law using a central piece of civil rights legislation, the 1965 voting rights act, which requires 16 states with a history of racially discriminatory laws and practices to clear all or some changes in voting laws and constituency boundaries with the justice department.

Texas brought this week's legal challenge in federal court on the grounds that the justice department has no authority to apply the voting rights act because the voter ID law was motivated solely by an attempt to prevent fraud.

The state said up to 50,000 dead people remain on the electoral roll. But the justice department said there is little evidence of significant illegal voting and that the state has only prosecuted one case in recent times.

John Hughes, acting for Texas, derided the administration's claim that as many as 1.5 million people who are registered to vote may not be able to.

"If that were remotely true the courtroom would be filled with people [without photo ID to vote]," he said. "It's hard to find people who don't have photo ID. Certainly the defendants didn't find anyone."

The justice department called only one witness who claimed to be directly affected by the law.

Victoria Rodriguez, 18, told the court that while her birth certificate and student ID were enough to permit her to board a plane, they wouldn't be sufficient voter identification under the new law. Lawyers for the state told the court Rodriguez could obtain the necessary ID using her birth certificate.

Texas constructed part of its case around social science studies that he said show voter ID laws have no effect on voting, he said.

"Common sense is people who want to vote already have ID or can get one," he said. "The evidence is that it is easy to get one in Texas and that's why it won't have a turnout impact."

Daron Shaw, a University of Texas professor of political science, said his research in states that already have voter ID laws suggests few people are kept from the polls by the requirement.

"I think the weight of the evidence is that it will not have an impact on turnout," he said.

The justice department challenged Shaw's objectivity on the issue, eliciting from him that he worked as a political strategist on both of President George W Bush's presidential campaigns.

Shaw's research also came under scrutiny from other witnesses who challenged the methodology of the study including that only 2% of those asked whether the law would affect them had responded.

As the case was heard, the political confrontation continued in Texas where the attorney general, Eric Holder, told a conference of the National Association for the Advancement of Coloured People (NAACP) that the justice department "will not allow political pretexts to disenfranchise American citizens of their most precious right".

Holder said that about one in four African Americans lacks government-issued photo identification of the kind required to vote in several states while fewer than one in ten white people do.

He said he opposed the Texas law because many people would have to travel too far to obtain the IDs, or would struggle to meet the cost of the necessary documentation.

The US supreme court in 2008 upheld the right of states to require voters to produce identification, but that case, involving Indiana, did not touch on the voting rights act.

Since then, 15 other states have passed laws requiring voters to show photo identification, although some of them offer alternatives. Texas has the most restrictive law.

© Guardian News and Media 2012

[Texas Voter ID via Vepar5 / Shutterstock]