NEW: The American Civil Liberties Union says law meant to reduce number of Democratic voters
Pennsylvania Supreme Court seeks further lower court review of voter ID law
The law requires most voters to show a photo ID before casting their ballots
The lower court had upheld the law in an August decision
(CNN) -- The Pennsylvania Supreme Court sent a controversial voter ID law back to a lower court on Tuesday to assess the availability of alternative forms of identification and whether the new law disenfranchises voters.
The high court said the state's Commonwealth Court has until October 2 to file its response, according to court documents.
The lower court, in August, upheld the law requiring that most voters show photo identification before casting ballots.
"Overall, we are confronted with an ambitious effort on the part of the General Assembly to bring the new identification procedure into effect within a relatively short time frame and an implementation process which has by no means been seamless in light of the serious operational constraints faced by the executive branch," the high court said Tuesday.
The law's opponents said the measure undermines the ability of registered voters to vote and that it was passed without sufficient evidence of prior identity fraud.
Its backers argue the law strengthens voting procedures and protects against fraud.
Gov. Tom Corbett, a Republican, signed the law in March after its passage largely along party lines. He said it "sets a simple and clear standard to protect the integrity of our elections."
Earlier, Pennsylvania Deputy Secretary of State Shannon Royer said, "It was clear by the lower court ruling that this law is absolutely constitutional."
But the American Civil Liberties Union and other advocacy groups say it is intended to reduce the number of Democrats at the polls.
"The Supreme Court was sufficiently nervous about whether all voters would in fact have ID on Election Day that they asked the Commonwealth Court to review the matter again," said Witold J. Walczak, legal director for the ACLU of Pennsylvania.