State elections officials voted Friday to keep a referendum against gay marriage off the November ballot, saying supporters failed to gather enough valid signatures.

The measure would have asked voters if the state constitution should be amended to ban gay marriage. Illinois law already prohibits same-sex marriage, but gay-marriage opponents say they fear courts could overturn the law unless the state constitution is changed.

"Unless they pull a rabbit out of a hat in federal court, it's not going to be on the ballot," said Patricia Logue, senior counsel for the Lambda Legal Defense and Education Fund.

Supporters of the referendum have filed a lawsuit in federal court, claiming that getting a referendum on the Illinois ballot is burdensome and unconstitutional, said Peter LaBarbera, executive director of the conservative Illinois Family Institute and Protect Marriage Illinois.

A district court judge dismissed the claim earlier this month and supporters now want a federal appellate court to intervene.

"It's not near over," LaBarbera said.

Amending the state constitution would be a lengthy process. The most common approach would be for both chambers of the Legislature to vote by three-fifths majority to put an amendment on the ballot, where it would have to be approved by three-fifths of voters.

Last month, elections officials said there weren't enough valid signatures in a sample check of some of the more than 330,000 signatures they had to consider for the petition. More than 95 percent of the sample's signatures must be valid; gay marriage opponents had 91 percent.