A statistical expert testified Tuesday in the trial on the constitutionality of Pennsylvania's voter ID law saying that hundreds of thousands of voters lack the photo identification the measure mandates.

Bernard Siskin, a Philadelphia consultant hired by the plaintiffs in the case, described his research in detail as the trial's first witness.

Siskin found that more than a half-million voters either lacked one of the three forms of photo IDs issued by the Pennsylvania Department of Transportation or possess IDs that expire before the Nov. 5 election.

A state consultant criticized Siskin's report in court papers, saying it exaggerates the problem, and Siskin says the consultant's methodology is faulty.

The plaintiffs hope to overturn the law backed by the Legislature's Republican majority and GOP Gov. Tom Corbett.

Pennsylvania's voter identification law would be one of the strictest in the nation if it is upheld.

Nine days have been set aside for the trial in Harrisburg in Commonwealth Court. Civil libertarians challenging the law and state officials defending it say they expect the state Supreme Court will ultimately decide the case.

After legal jousting that reached the state Supreme Court, Judge Robert Simpson blocked enforcement in last year's presidential election and again in this year's municipal and judicial primary because of lingering concern that it could disenfranchise voters who lacked a valid photo ID.

The 2012 law was approved without any Democratic votes by the Legislature's Republican majority and signed by Corbett amid a bitterly contested White House race in which Democratic President Barack Obama ultimately carried Pennsylvania and was re-elected.

Critics derided the law as a cynical GOP effort to discourage voting by young adults, minorities, the elderly, poor and disabled from going to the polls. Republicans said most Pennsylvanians have driver's licenses to use as photo ID and claimed that the law would discourage voter fraud.

The judge's verdict may be reviewed by a Commonwealth Court panel before an inevitable appeal to the state Supreme Court by the losing side.

Plaintiffs in the case include the Pennsylvania League of Women Voters, the NAACP and the Philadelphia-based Homeless Advocacy Project.

A key issue in the trial will be the availability of alternative photo identification for people who lack a driver's license or other types of acceptable ID listed in the law. The Department of State has developed a special photo ID that is available free to voters who have run out of other options.

Lawyers for the plaintiffs say fewer than 20,000 such IDs have been issued so far, but many more voters still lack valid credentials. State officials say they have made the special cards easily accessible and anyone who does not have valid identification must not want it.

Based on reporting by The Associated Press.

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