A unanimous U.S. Supreme Court on Monday sent a clear message that plaintiffs bringing discrimination lawsuits under key civil rights laws now will face a high bar to winning federal court review of their claims.

That message, however, came with a separate warning from Justice Ruth Bader Ginsburg that the 1866 law at issue before the justices was "sweeping" and designed to "break down all discrimination between black men and white men" regarding "basic civil rights." Attempts to "cabin" its scope, as advanced by the Trump administration's Justice Department and others, "cannot be squared with the statute."

The case before the justices was a multibillion-dollar contract discrimination case brought by the National Association of African American Owned Media and Entertainment Studios Network against Comcast Corp. The justices ruled that a plaintiff who sues under 42 U.S.C. §1981, which guarantees an equal right to make contracts, must both plead and prove that race was the "but-for" cause of the injury.

"This ruling weakens our nation's oldest civil rights statute and may shut the courthouse door on some discrimination victims who, at the complaint stage, may simply be without the full range of evidence needed to meet the court's heightened standard," Kristen Clarke, president and executive director of the Lawyers' Committee for Civil Rights Under Law, said in a statement Monday.

With the exception of Ginsburg's separate concurrence, her colleagues on the left joined the decision without comment. Those colleagues' silence may reflect the reality that the high court, often in 5-4 decisions, in recent years has been applying the "but-for" causation test to civil rights laws where Congress has not explicitly imposed a different standard, and that standard is now, as Ginsburg conceded, "the default rule."

Business advocates, meanwhile, applauded the decision. Cory Andrews, Washington Legal Foundation's vice president for litigation, said the unanimous decision "restores common sense and uniformity to the pleading threshold for discrimination claims under §1981."

Gibson, Dunn & Crutcher partner Miguel Estrada argued on behalf of Comcast. Erwin Chemerinsky, dean of the University of California Berkeley School of Law represented Entertainment Studios Network.

The Supreme Court, in an opinion by Justice Neil Gorsuch, rejected arguments by Entertainment Studios Network and the U.S. Court of Appeals for the Ninth Circuit that a plaintiff must only plead facts plausibly showing that race played "some role" or motivating factor in the defendant's decision-making process.

The justices vacated the Ninth Circuit decision and remanded it for application of the correct causation standard.

Entertainment Studios Network sued Comcast after negotiations failed to persuade the cable television company to carry the African American-owned television network's programming. Comcast cited lack of demand for the programming, bandwidth constraints and preference for programs that the network did not provide. The network alleged Comcast disfavored "100% African American-owned media companies."

Gorsuch said the rule requiring a plaintiff to establish causation is well established. "In the law of torts, this usually means a plaintiff must first plead and then prove that its injury would not have occurred 'but for' the defendant's unlawful conduct," he wrote. "The plaintiffs before us suggest that 42 U.S.C. §1981 departs from this traditional arrangement. But looking to this particular statute's text and history, we see no evidence of an exception."

This burden on the plaintiff, he added, remains constant from filing to judgment.

The justices declined to rule on whether the statute addresses the whole contracting process, not just outcomes—the right to contract, sue, be a party, and give evidence—as Comcast and the Justice Department argued. Gorsuch said answering that question was not needed to resolve the case.

Ginsburg made two points in her concurring opinion. First, she said, a strict "but-for causation" standard is "ill-suited to discrimination cases and inconsistent with tort principles." Ginsburg, however, said she recognized the standard is now the "default rule" for anti-discrimination laws.

Second, Ginsburg criticized Comcast's view of §1981. That view, she wrote, "countenances racial discrimination so long as it occurs in advance of the final contract-formation decision." That view, she said, would allow an employer to reimburse expenses for white interviewees but require black applicants to pay their own way. And, citing the government's amicus brief, an employer could "refuse to consider applications" from all black applicants without violating the law.

"Far from confining §1981's guarantee to discrete moments, the language of the statute covers the entirety of the contracting process," she wrote.

In a statement, Comcast said "the Supreme Court unanimously restored certainty on the standard to bring and prove civil rights claims. The well-established framework that has protected civil rights for decades continues. The nation's civil rights laws have not changed with this ruling; they remain the same as before the case was filed."

Chemerinsky called the Comcast ruling a "terrible" decision for civil rights. "It is a very difficult standard to meet," he said referring to but-for causation. "I think the battle will now shift to Congress to amend section 1981 and perhaps other civil rights statutes to allow claims to go forward on a showing that race is a motivating factor in the denial of a contract."