LOS ANGELES -- At a time when election officials are struggling to convince more Americans to vote, advocates for the disabled say thousands of people with autism spectrum disorder, cerebral palsy and other intellectual or developmental disabilities have been systematically denied that basic right in the nation's largest county.

A Voting Rights Act complaint to be filed Thursday with the U.S. Justice Department goes to a politically delicate subject that states have grappled with over the years: Where is the line to disqualify someone from the voting booth because of a cognitive or developmental impairment?

The complaint by the Disability and Abuse Project argues that intellectual and developmental disabilities, including conditions such as Down syndrome, are not automatic barriers to participating in elections. It seeks a sweeping review of voting eligibility in Los Angeles County in such cases, arguing that thousands of people with those disabilities have lost the right to vote during the last decade.

"We want these past injustices to be corrected, and we want the judges and court-appointed attorneys to protect, not violate, the rights of people with developmental disabilities," Thomas F. Coleman, the group's legal director, said in a statement.

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At issue in the California case is access to the ballot box for adults who enter so-called limited conservatorships, legal arrangements in which parents or guardians assume the right to make certain decisions for people who lack the ability to manage their financial and medical affairs. In the course of taking that step in court, voting rights are routinely voided, according to the advocacy group.

California has over 40,000 such cases, and those covered by the arrangements usually live with their families or in group homes. A recent sample of 61 cases by the advocacy group in Los Angeles County found that 90 percent of the people covered by limited conservatorships had been disqualified from voting.

The complaint says judges in Los Angeles Superior Court use literacy tests to determine if adults in limited conservatorships should have voting rights, a violation of the federal Voting Rights Act. It also says that judges and court-appointed attorneys violate federal laws that allow people with disabilities to have assistance to complete voter-registration forms and cast ballots.

"Autism is a broad spectrum, and there can be low skills and there can be high skills. But what I observed was that people tend to just dismiss it as though they have no skills," Teresa Thompson, whose son has autism and whose case helped prompt the complaint, said in a videotaped statement.

Los Angeles Superior Court spokeswoman Mary Eckhardt Hearn said she had not seen the complaint and declined comment.

The complaint could trigger an investigation by the Justice Department. It also asks Superior Court to rescind thousands of voter-disqualification notices it has issued in those cases over a decade.

For years, advocates brought attention to the obstacles to voting faced by the physically disabled. More recently, the focus has shifted to the mentally or developmentally disabled, who advocates say have long been stigmatized in the voting process.

In the past, advocates in Missouri sued to make it easier for people under guardianship for mental disabilities to vote, and New Jersey voters in 2007 stripped language from the state Constitution that held "no idiot or insane person shall enjoy the right of suffrage."

All but about a dozen states have some type of law limiting voting rights for individuals based on competence. But how those laws are enforced varies widely, advocates say.

A 2007 report for the American Bar Association concluded that "excluding the broad and indefinite category of persons with mental incapacities is not consistent with either the constitutional right to vote ... or the current understanding of mental capacity."

The California complaint could create a testing ground for such cases. State election law says a person is considered mentally incompetent and disqualified from voting if he or she cannot complete a voter-registration form, which the complaint argues is an illegal literacy test.

"There is this constant struggle to make sure everyone can vote privately and independently, regardless of disability," said Curtis Decker, executive director of the National Disability Rights Network.