WASHINGTON - House Assistant Democratic Leader Jim Clyburn and voting-rights advocates warned Wednesday that laws in South Carolina and other states could disenfranchise millions of Americans in the November presidential elections.

Clyburn compared the voter photo ID law that Gov. Nikki Haley signed last month and similar laws in other states with the post-Reconstruction Jim Crow laws Southern states enacted that imposed poll taxes, literacy tests and other hurdles that prevented blacks from going to the polls.

"I liken what's going on today to that same period of time," Clyburn said Wednesday. "It was effective then, and if we aren't vigilant, it will be effective today. We must make sure that people are aware of the danger to our democracy."

South Carolina's law, which requires voters show a driver's license, state ID card, passport or U.S. military credentials, is blocked by the Justice Department and ensnared in a state lawsuit against the federal government.

S.C. Rep. Alan Clemmons, a Myrtle Beach Republican who crafted the state law, defended it.

"Absolutely in no way is our voter ID law discriminatory, even though it has been challenged by (President) Barack Obama's Justice Department," Clemmons said.

"Any ID that will get you on an airplane in the United States will get you into a voting booth in South Carolina," he said.

A new report by the Center for American Progress, a liberal think tank in Washington, singled out recent voting ID laws in South Carolina, Texas and Tennessee for criticism because they don't allow young people to use student ID's at state-run colleges and universities.

The report said one-quarter of African-Americans don't possess the types of photo identification cards required by such laws, compared with 11 percent of all Americans.

The group named Florida, Texas, Kansas, Tennessee and Wisconsin as the five worse voting-rights states, citing their rollbacks of early-voting periods, limits on voting by ex-felons and restrictions on the ability of groups such as League of Women Voters to register people.

"There is an explosion in the number of states considering these retrograde disenfranchisement rules," said Tom Perriello, head of the center.

Last December, the Justice Department rejected South Carolina's then-proposed voter ID law, saying it would hinder African-Americans' ability to vote because one-third of black registered voters don't have a driver's license.

Under the 1965 Voting Rights Act, the Justice Department oversees all election-related laws and regulations in South Carolina and eight other Southern states with a history of racial discrimination.

South Carolina filed a lawsuit in February, asking the federal courts to overturn the DOJ decision.

S.C. Attorney General Alan Wilson, who filed suit on behalf of the state, said the law isn't discriminatory.

"It doesn't disenfranchise anybody of anything," he said. "It treats everyone equally.

"People could still show up on Election Day and sign a sworn affidavit saying they are who they say they are, and they would be allowed to vote. So there's a safety net there."

After Georgia implemented a voter photo ID law in 2006, Wilson said, blacks' participation in the November elections increased from 513,700 in 2006 to 741,000 in 2010.

Wilson said the state Wednesday filed a motion accepting requests by the NAACP and the League of Women Voters to intervene in the case on the side of the Justice Department.

"We welcome them," Wilson said. "We think they have a right to be heard on this issue, so long as it doesn't impede the courts' ability to hear this case in a timely manner."