The immigrant advocacy group Casa de Maryland asked a court Monday to toss out the referendum aimed at overturning the new law that extends in-state tuition breaks at Maryland's public colleges and universities to illegal immigrants.

The widely expected filing is now the highest remaining hurdle confronting opponents of the breaks, who capitalized on popular support and previously untested technology to gather nearly twice the number of signatures they needed to get the passion-stirring law on the 2012 ballot.

Such lawsuits have worked in the past: The last petition to be certified by the State Board of Elections was nullified when a court found evidence that Maryland's strict signature-gathering rules were not followed.

In the suit against the elections board filed Monday at Anne Arundel County Court, CASA focuses on the sophisticated website that opponents developed to collect the signatures. The organization says fraudsters could use the site to print thousands of online petition forms and provide fake signatures with no way of getting caught.

"There is absolutely no procedure or step … that could detect the fraud," according to the lawsuit.

The suit was filed by attorneys working pro bono on behalf of Casa and eight plaintiffs, including two illegal immigrants who are not named. Several of the plaintiffs gathered in front of the Anne Arundel County Circuit Courthouse on Monday morning to express support for the law.

"I find it very disturbing that we are targeting children," said Catherine Brennan, a plaintiff from Baltimore. She said she signed on to the lawsuit because she believes her children's education will be enhanced with the exposure to undocumented students.

Del. Neil C. Parrott, the Republican freshman who spearheaded the petition drive against the law, characterized the lawsuit as "frivolous" and "outrageous" and said Casa was "grasping at straws."

"They don't want to allow Marylanders to vote on this in 2012," Parrott said. "They realize that if Marylanders vote on it, they will say 'No.'"

The State Board of Elections will be represented by a lawyer in the office of Maryland Attorney General Douglas F. Gansler. A spokesman for Gansler declined to comment.

The petition drive proved to be the most popular in recent state history, attracting broad bipartisan support with signatures from 63,487 Republicans, 32,397 Democrats and 12,628 independents.

The law, which is now suspended pending the referendum, would provide undocumented students the same in-state tuition discounts available to legal residents. To be eligible, students would have to attend three years of high school in Maryland and show that their parents had filed tax returns to the state.

It was one of the most contentious issues in Annapolis this year, passing only in the final hours of the legislative session, after Senate Republicans backed down from a filibuster attempt.

Days after Gov. Martin O'Malley signed the bill into law in April, Parrott put down about $5,000 of his own money to launch the petition campaign.

The delegate received pro bono help designing MDPetitions.com, the website that walks users though a multistep process for filling out, printing and mailing a petition form.

Users type their names into an online form, which automatically compares them against a list of registered voters to reduce errors of the sort that can get a petition thrown out. Under Maryland's strict rules, names on petitions must be identical to those on the voter registration rolls.

Parrott has said he plans to keep the website running for possible use against future legislation passed by the Democratic General Assembly.

But Casa and its allies argue that state election law requires real people — not computers — to ink names and addresses onto the petition forms.

Casa describes it thusly: "The majority of … invalid signatures were purportedly of voters whose information was not filled out by the voter herself, as required by law, but was instead filled out by a computer program operated by the petition sponsors."