Attorney General Jeff Sessions has said the administration will not defend parts of the Affordable Care Act in a lawsuit challenging the law’s constitutionality. (Mike Moore/Journal Gazette/AP/Mike Moore/Journal Gazette/AP)

A broad swath of health-care constituencies weighed in on Thursday to oppose a lawsuit challenging the Affordable Care Act, forming an uncommonly united front against a decision by the Trump administration not to defend significant parts of the law.

Hospitals, doctors, medical schools, patient-advocacy groups, the health insurance industry and others filed briefs in a federal court in Texas, disputing the argument of 20 Republican-led states and the Justice Department that all or part of the 2010 law is unconstitutional.

In all, 11 friend-of-the-court briefs were filed. From various vantage points, each argues that a ruling in favor of this latest challenge to the ACA’s constitutionality would “have a devastating impact on doctors, patients, and the American health care system as a whole,” as a brief from the American Academy of Child and Adolescent Psychiatry puts it.

Striking down the law “would strip health coverage and protections from nearly 30 million people” and “have catastrophic economic consequences,” argues a brief from the Service Employees International Union, which says it represents more than 1 million health-care workers.

Ending the law “would cause an enormous surge” in uninsured Americans, the SEIU says, leaving hospitals and other providers of care with an estimated $1 trillion worth of services for which no one paid the bills. Millions of health-care jobs would be lost as a result, the brief contends.

One brief in support of the lawsuit was filed last month by a coalition of gun-owners’ groups and Citizens United, a conservative organization that won a landmark Supreme Court case freeing corporations and unions to spend money on political campaigns. The coalition echoes the Republican states’ argument that the ACA is now unconstitutional because Congress late last year repealed the tax penalty the law imposed on Americans who flout its requirement to carry health insurance.

Thursday’s filing deadline for outside briefs came a week after Attorney General Jeff Sessions staked out the administration’s bold legal position, which he said President Trump had approved.

That stance does not go as far as that of the attorneys general of Texas and the other GOP-led states, who are arguing that the entire law is now unconstitutional.

Instead, Justice contends that ending the penalty at the start of 2019 will make the ACA’s insurance requirement unconstitutional. And the mandate, the department argues, cannot be legally separated from a variety of consumer insurance protections, including ones for people with preexisting medical conditions.

The government’s position, contrary to Trump’s promises to preserve such popular protections, surprised even the administration’s allies in Congress and kicked up a political ruckus.

Democrats have begun wielding the reversal on consumer protections as a club in advance of the November midterm elections. And congressional Republicans have distanced themselves from the administration’s move. “Everybody I know in the Senate — everybody — is in favor of maintaining coverage for preexisting conditions,” Senate Majority Leader Mitch McConnell (R-Ky.) said this week.

Groups that filed briefs Thursday include the trade group America’s Health Insurance Plans, which wrote that the suing GOP-led states “seek to turn off the health insurance system as we know it with the flip of a switch.” If the law were struck down, AHIP says, tens of millions of Americans covered through Medicaid and Medicare would be affected, as would those with the subsidized private insurance the ACA created for people who cannot get affordable health benefits through a job.

Also weighing in were three hospital trade groups, the Association of American Medical Colleges, and five major groups advocating for patients with diseases, including the American Cancer Society, the American Heart Association and the American Lung Association. The liberal ­consumer-health lobby Families USA, the Small Business Majority, a group of law professors and a separate group of health-care and economic researchers also filed briefs.

amy.goldstein@washpost.com

Julie Tate contributed to this report.