HARRISBURG - Critics of the month-old voter-identification law are poised to challenge it in the courts and General Assembly.

The American Civil Liberties Union says it will file suit over the law's constitutionality by the end of April, and two Philadelphia Democrats are set to introduce a bill Tuesday that would repeal the controversial measure.

"There is no basis for the law in the first place. No clear fraud across the state was ever demonstrated," said Rep. Dwight Evans, who is to appear with Rep. John Myers at a news conference Tuesday at the Pennsylvania Department of Transportation office at 7121 Ogontz Ave. in West Oak Lane.

The ACLU, along with the NAACP, says it will sue on grounds the law discriminates against voters, especially the elderly, the poor, urban residents, and the disabled.

"It will result in the disenfranchisement of thousands of legal Pennsylvania voters in order to combat virtually nonexistent voter fraud," said Witold Walczak, legal director of the ACLU of Pennsylvania.

The NAACP is lining up plaintiffs for the suit, said J. Whyatt Mondesire, president of the Philadelphia branch of the NAACP.

Gov. Corbett, who signed legislation in March making Pennsylvania the 16th state with a photo-ID requirement for voting (although several of those are being challenged and others have yet to take effect), said the law was aimed at protecting voters' rights.

Ron Ruman, spokesman for Secretary of State Carol Aichele, said Monday that he was not aware of a challenge to the law, but that the administration would defend the law.

Under the law, the Pennsylvania Department of Transportation will provide nondriver IDs to any Pennsylvanian who applies for one, providing he or she has a valid birth certificate.

The cost of waiving the $13.50 ID fee is estimated at between $4 million and $11 million. There's such a wide disparity in the estimates because it is not known how many Pennsylvanians will be affected by the law.

Voters will be asked to show a photo ID in the April 24 primary election but will not be turned away if they don't have one.

Mondesire said the NAACP was helping pay costs incurred by some senior citizens who need to get out-of-state birth certificates, particularly those who were unable to get such documents decades ago because of discriminatory restrictions in the Jim Crow-era South.

The ACLU has not determined yet in which court it will file the lawsuit, Walczak said.

Cases have been heard in federal and state courts in at least four Republican-controlled states that recently enacted voter-ID laws.

Last month, the U.S. Justice Department blocked Texas from enforcing a photo-identification law.

At the same time, a Wisconsin state judge ruled that requiring a photo ID to vote was unconstitutional.

PennDot officials say they have so far issued 600 nondriver IDs for voting purposes since Corbett signed the bill March 14.

"So far, so good," said Janet Dolan, PennDot director of the bureau of driver licensing.

But Walczak said he feared PennDot was underestimating the number of Pennsylvanians without driver's licenses by hundreds of thousands.

"The huge issue that's difficult to capture until Election Day is that people who aren't plugged in but who vote regularly and show up without ID and can't vote is going to be in the thousands," he said. "This will wreak havoc on the election in November."

Mondesire said the Republican-crafted Pennsylvania law was designed to suppress the Democratic vote in an election year when a Democratic president is seeking reelection.

"Nationally, we see this as a gross and grotesque attack on the voting rights of African Americans, Latinos, the recently incarcerated, and the young, who have the most difficulty getting photo ID," Mondesire said. "We will resist this in every way we can."

Asked about the prospects of repealing the law when Democrats failed to line up votes to block passage of it, Evans said he believed the public, once it learned of the problems with the law, would pressure the legislature to reverse course.

"It's time for the public to weigh in," said Evans, who is launching a public-awareness campaign about the law. "Fundamentally, we believe the law is wrong."

Contact Amy Worden at 717-783-2584, aworden@phillynews.com, or follow @inkyamy on Twitter.