Supporters of the Affordable Care Act protest during a rally at Burnett Park in Fort Worth, Texas, Wednesday, Sept. 5, 2018. Democratic nominee for Texas Attorney General Justin Nelson hosted the Fort Worth Rally for Preexisting Coverage Protection. (Max Faulkner/Star-Telegram via AP)

Supporters of the Affordable Care Act protest during a rally at Burnett Park in Fort Worth, Texas, Wednesday, Sept. 5, 2018. Democratic nominee for Texas Attorney General Justin Nelson hosted the Fort Worth Rally for Preexisting Coverage Protection. (Max Faulkner/Star-Telegram via AP)

AUSTIN, Texas (AP) — The latest push to scrap the Affordable Care Act once and for all pressed ahead Wednesday as Republican-controlled states asked a federal judge to finish what Congress started last year and bring the law that insures 20 million Americans to a halt.

A small group of protesters, some holding signs reading “Save the ACA,” shouted across the street from a Fort Worth, Texas, courthouse where former President Barack Obama’s health care law is again under attack. At issue are core principles of the law, including protections for people with pre-existing medical conditions and limits on how much older customers can be charged.

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U.S. District Judge Reed O’Connor made no immediate ruling following a four-hour hearing. Twenty GOP-led states brought the lawsuit, arguing that the entire health care law was rendered unconstitutional after Congress repealed the “individual mandate” that required most Americans to buy insurance or risk a tax penalty.

“Texans and other Americans should be free again to make their own health care choices,” said Republican Texas Attorney General Ken Paxton, who is leading the court challenge.

The case is the most high-profile legal challenge to “Obamacare” under President Donald Trump, whose administration is not defending the law in court. But the Justice Department doesn’t want an immediate injunction suspending enforcement of the law, even as Republicans press for one.

Justice Department attorney Brett Shumate told the judge that any immediate injunction could create “a potential for chaos,” The Dallas Morning News and the Fort Worth Star-Telegram reported.

The arguments in Texas unfolded as senators in Washington pressed Supreme Court nominee Brett Kavanaugh, whose confirmation could swing the next major challenge to the health care law. Kavanaugh issued a 2011 opinion that some conservatives viewed as favorable to the individual mandate, but Democrats worry he will provide a key vote on the court against the law.

The Trump administration sitting out the case has left defense of the law up to Democratic state attorneys general. On Capitol Hill, Democrats sought to tie the Texas case into the fight over whether Kavanaugh should serve on the Supreme Court.

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“The Republican-backed lawsuit that seeks to take away protections from people with pre-existing conditions makes the stakes as high as could be,” Senate Minority Leader Chuck Schumer, D-N.Y., said at a news conference.

The law has proved largely durable so far — both in previous challenges that reached the U.S. Supreme Court and in a failed GOP effort to repeal it during Trump’s first year in office.

But the collation of GOP-controlled states saw a new opening when Congress removed the individual mandate as part of a tax overhaul last year. Paxton has said that without the tax penalty, there is no “remaining legitimate basis for the law.”

William Sage, a professor of law and medicine at the University of Texas at Austin, called it a “swing for the fences lawsuit” that appeared thinner than previous challenges.

Nathan Cortez, a law professor at Southern Methodist University in Dallas, said it’s not clear whether this challenge is different from other attempts to strike down the law that have failed.

“All it takes is for a district court judge and then an appellate panel to find some validity in the argument for it to get up to the Supreme Court,” he added.

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Associated Press writers Jamie Stengle in Dallas and Kevin Freking in Washington contributed to this report.

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This story has been corrected to reflect that the University of Texas at Austin professor’s first name is William instead of Williams.