HARRISBURG - The legal and political dispute over Pennsylvania's voter identification law showed every sign of widening into a brawl Wednesday.

Civil liberties groups suing to upend the law contended in court briefs that as many as a million or more Pennsylvanians don't have the types of photo ID the law requires in order to vote in this fall's presidential election.

They also claimed the state had fueled suspicion of partisan motives by conceding it cannot pinpoint a single instance of the type of voter-impersonation fraud the law aims to prevent.

Their brief landed on the same day a Republican election official in Philadelphia was claiming to have unearthed evidence of voting irregularities that the new law could have prevented.

"The best antidote to voter suspicion would be for the commonwealth to publicly acknowledge what it has confessed . . . namely, that in-person voter fraud is nonexistent in Pennsylvania," wrote lawyers for the American Civil Liberties Union's state branch along with the Advancement Project, the Public Interest Law Center of Philadelphia, and the Washington law firm of Arnold & Porter LLP.

The groups noted that the law was passed earlier this year by the legislature along party lines - Republicans for, Democrats against - and that last month, state House Majority Leader Mike Turzai had told a meeting of fellow Republicans the law would help GOP presidential candidate Mitt Romney win Pennsylvania on Nov. 6.

"The only fair inference is that the real purpose of the Photo ID Law is not ensuring the integrity of the electoral process, but ensuring political advantage through the exclusion of qualified voters who are perceived supporters of the opposition," the attorneys wrote.

But even as they were preparing to file their brief, City Commissioner Al Schmidt, the sole Republican on the board that oversees Philadelphia elections, announced that an investigation of the April primary found evidence of voting irregularity and fraud - and that the new law could have helped prevent such problems.

The dueling positions on the necessity and purpose of the voter ID law underscored just how controversial it has become - and not just in Pennsylvania. Nationwide, Democrats have been screaming for months that the effort to require voters to present photo identification at the polls will disenfranchise the poor, the elderly, and those in urban areas.

In its brief, the ACLU estimated that a third of Pennsylvania's 8.3 million voters were still unaware of the new law.

The group also contended that the Corbett administration's estimate of voters who don't have state-issued photo ID - about three quarters of a million people - was too low. It said that figure excluded voters such as those who have expired driver's licenses.

"The universe of registered voters who lack a valid PennDot ID is closer to 1.4 million," the ACLU brief said.

The ACLU lawyers are suing on behalf of 10 Pennsylvanians who don't have drivers' licenses who contend the photo ID requirement violates the state Constitution and deprives them of their right to vote.

Hearings in the widely watched case are scheduled to begin Wednesday in Commonwealth Court.

Lawyers for the state, in a response also filed Wednesday, countered that the new law will not disenfranchise anyone, and that the 10 plaintiffs have plenty of opportunity between now and Nov. 6 to obtain acceptable ID.

Under the new law, various other forms of photo identification will be accepted at the polls in the fall, including valid U.S. passports, college student identification cards that have expiration dates, current military identification, and ID cards issued to government employees.

In addition, PennDot will issue non-drivers' photo identification cards free to those who apply. Critics have countered that even those IDs will require some residents to dig up a birth certificate and Social Security card - and that finding those documents may be time-consuming, costly, and a hardship for those with disabilities, especially if they live far from a PennDot center.

The state's lawyers said the process was not burdensome, and that applicants' family members and friends could help by transporting the would-be voters to a PennDot center to obtain an ID.

They also argued that the legislature had the authority and wide discretion to regulate voting. Among other cases, they cited a decision upholding the denial of voting rights to prison inmates. That decision, the lawyers said, was well within constitutional bounds even though it "explicitly and systematically disenfranchised an entire swath of the population."

Meanwhile, Secretary of State Carol Aichele, who oversees Pennsylvania elections, said Wednesday the state has "a few other ideas that we're working on."

Aichele, speaking to reporters after a meeting in Gettysburg where she explained the ID law to organizations that work with the elderly, declined to elaborate.

But the ACLU's brief offered a possible clue to those "other ideas," saying documents show that the state's lawyers have been urging an entirely new form of photo ID to be developed and issued by the Department of State, which Aichele heads.

Those documents, according to the ACLU brief, say the new card would be announced Tuesday - on the eve of court hearings in the lawsuit.

Reached for comment Wednesday, Department of State spokesman Nick Winkler would not confirm any such plan. He did say "there will be an announcement" - but not on Tuesday.

"I don't think it's significant as you think," Winkler said. "For the vast majority of Pennsylvanians, the process of obtaining an acceptable photo ID under the new law has been simplified and fast-tracked."

Contact Angela Couloumbis at 717-787-5934, acouloumbis@phillynews.com, or follow on Twitter @AngelasInk.