By Jon Herskovitz

AUSTIN, Texas (Reuters) - Some Texas voters may need to show a state postcard listing them on the election roll to cast ballots in November elections after a U.S. appeals court found the state's voter ID was discriminatory, specialists said on Thursday.

The U.S. Court of Appeals for the Fifth Circuit on Wednesday ordered a remedy ahead of the November general election to the law, which requires voters to present a photo identification such as a Texas driver's license, passport or military ID card.

Challengers said the Texas law discriminated against poor people and minorities such as Latinos and African-Americans, up to 600,000 of whom would be unable to vote if the law was fully in effect.

Allowing a voter registration certificate as an acceptable identification is in the realm of possible remedies, Deuel Ross, an attorney for the NAACP Legal Defense and Educational Fund who helped litigate the case, said on Thursday.

He said there may also be a reasonable impediment exemption where some voters sign an affidavit saying they have had trouble obtaining a mandated ID.

"We think all of those are on the table right now as things that judges are very seriously considering," he said.

A U.S. district judge this month put in place similar requirements in Wisconsin, which had a voter ID law along the lines of the one in Texas.

The addition of these voters in a state with 27 million people will likely have little effect on the presidential race, where Republican nominee Donald Trump is expected to win in the longtime Republican stronghold, said Mark Jones, a professor in political science at Rice University in Texas.

But the court's decision could affect four or five local races, he added.

The Texas governor and secretary of state, named as defendants in the case, did not respond to requests for comment.

Critics of the law said it and similar Republican-backed statutes were intended to make it harder for minorities, who tend to support Democrats, to vote. Backers of these laws have said they are necessary to prevent voter fraud.

The Texas measure was signed into law in 2011, and subject to lawsuits after that.

The Fifth Circuit said there was little time to implement a remedy and sought a fix from a U.S. district court, which has ruled the current ID law discriminates against minorities who can have trouble obtaining the mandated IDs.

(Reporting by Jon Herskovitz; Editing by David Gregorio)