A federal judge Tuesday blocked Georgia from enforcing a new state law requiring voters to show photo identification at the polls.

In issuing the preliminary injunction, U.S. District Judge Harold Murphy (search) said the law amounts to an unconstitutional poll tax because the state is not doing enough to make ID cards available to those who cannot afford them.

The requirement "is most likely to prevent Georgia's elderly, poor and African-American voters from voting," Murphy wrote. "For those citizens, the character and magnitude of their injury — the loss of their right to vote — is undeniably demoralizing and extreme."

So far, the law has been used only for local elections. The injunction could prevent its use during municipal elections Nov. 8.

Voter and civil rights groups sued over the new law, which eliminates the use of other forms of voter identification, such as Social Security cards, birth certificates or utility bills. Supporters, including Republican Gov. Sonny Perdue (search), argued that the measure would help prevent fraud.

A driver's license with a photo is sufficient under the law. But those who do not have a license must obtain a state ID card, which can cost up to $35. The governor said such cards would be given free to those who cannot afford the fee.

The GOP-backed measure heightened racial tensions during the legislative session. Most of Georgia's black lawmakers walked out of the Capitol when it passed in March, some loudly singing a civil rights-era protest song. The widow of the Rev. Martin Luther King Jr., Coretta Scott King, called on the governor to veto the bill.

State Rep. Tyrone Brooks (search) called the judge's ruling "a strong signal from the federal judiciary that Georgia has gone too far in impeding the right of citizens to vote."

Supporters of the law said they would challenge the ruling.

"We'll appeal it until the Supreme Court makes a decision. Hopefully by then the president will have a good conservative court up there that understands the will of the people," said Republican state Sen. Don Balfour.

The law did not go into effect until August, when it was approved by the U.S. Justice Department. Under the Voting Rights Act, Georgia and other states with a history of suppressing minority voting must get the Justice Department's permission to change their voting laws.

Nineteen states require voters to show identification, but only five request photo ID, according to the National Conference of State Legislatures. Those states — Arizona, Florida, Louisiana, South Carolina and South Dakota — allow voters without a photo ID to use other forms of identification or sign an affidavit of identity.