EXCLUSIVE: Two Republican lawmakers are leading 18 members of the U.S. Senate and 64 members of the U.S. House of Representatives in filing an amicus brief at the Supreme Court against selective abortions based on a fetal Down syndrome diagnosis.

Sen. Tom Cotton, R-Ark., and Rep. Ashley Hinson, R-Iowa, are leading over 80 of their colleagues in a brief defending an Arkansas law designed to protect the unborn with Down syndrome from "invidious discrimination," according to the court filing exclusively obtained by Fox News.

The bicameral group of lawmakers is asking the court to overturn a lower court ruling that made abortions based on Down syndrome diagnoses legal, warning that the ruling could send the nation down the dark path of eugenics – the practice of selectively breeding people for desired traits.

The lower court ruling upheld an injunction on a 2019 Arkansas law that outlawed aborting a fetus based on a Down syndrome diagnosis. This ruling means that, currently, women in Arkansas can legally abort their child if it has been diagnosed with the chromosomal disorder.

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"Every life is valuable and has dignity and our laws should reflect that," Hinson told Fox News. "I am proud to lead this brief with Sen. Cotton to ensure that abortions are never performed based on a Down syndrome diagnosis."

"I will always defend the most vulnerable and stand up for the unborn," she added.

Cotton told Fox News that America must protect the unborn, including those with disabilities.

"Our society has an obligation to protect the most vulnerable, including unborn children with disabilities," the Arkansas senator said. "Arkansas' law seeks to protect babies with Down syndrome from modern-day eugenicists who want to end their lives, simply because of their disability."

"We stand with Arkansas and the unborn, and we will fight to uphold this law at the Supreme Court," added Cotton.

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The brief argues that the Arkansas law "aims to combat invidious discrimination by preventing doctors from performing selective abortions based on a Down syndrome diagnosis."

"Performing an abortion on the basis of a disability disregards the dignity of those with disabilities and perpetuates abortion as ‘a tool of modern-day eugenics,’" the brief reads.

The lawmakers also argue that Congress has long fought for the rights of people with disabilities, even the unborn, with legislation such as the American Disabilities Act.

They also said that the Arkansas law follows Congress’ "overarching federal policy of nondiscrimination" and that aborting fetuses based on a disability like Down syndrome would be inherently discriminatory.

Rep. Pete Sessions, R-Texas, who is a father of a son with Down syndrome, also signed onto the brief. 

"I am blessed to be a father of Down syndrome young man, Alex—he is one of the greatest joys in my life. The United States of America grants rights to an individual based on their God-given existence, with or without disabilities. Alex and many others are proof of the greatness Down syndrome individuals can accomplish and the value they bring to society," said Sessions. 

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Also on Thursday, Missouri Attorney General Eric Schmitt led a coalition of 22 states in filing an amicus brief in support of the Arkansas law.

Despite support from pro-life groups and lawmakers, other organizations such as the American Civil Liberties Union are preparing to challenge the law because they argue that it would open the door for the court to overturn the landmark abortion decision, Roe v. Wade. 

An amicus brief is an argument presented to the court by a party not affiliated with the case.

The brief was signed onto by many prominent Republican lawmakers, including Sens. Marsha Blackburn, R-Tenn., and Marco Rubio, R-Fla., as well as Reps. Kevin Brady, R-Texas, and Jim Banks, R-Ind.