WASHINGTON — The Obama administration's push to apply 1960s and 1970s equal rights laws to the transgender community is likely to win favor in federal courts and may even get a surprising assist from the late conservative Supreme Court Justice Antonin Scalia.

Transgender rights emerged this week as the new frontier in civil rights and the latest battleground for the nation's culture wars.

The Obama administration's Education and Justice departments on Friday sent a letter to public school officials nationwide, warning them that schools must provide an environment free of discrimination and treat students consistent with their “gender identity,” even if that's different from the sex assigned at birth.

Earlier this week, the Justice Department and North Carolina's Gov. Pat McCrory filed dueling lawsuits against each other, asking a judge to decide whether H.B. 2, the state's so-called bathroom bill, violates federal civil rights laws.

The bill says public bathrooms and changing facilities may be used only “by individuals based on their biological sex (as) stated on a person's birth certificate.”

At issue in all these cases is a deceptively simple question: What is discrimination based on “sex”?

Neither Congress nor the Supreme Court has weighed in directly to decide whether federal law forbids discrimination against transgender students or adults.

But Obama administration officials in recent years have put employers and schools on notice that they think existing law protects transgender people.

To bolster their argument, attorneys for the Obama administration argue the ban on sex discrimination has evolved since the 1960s, when it was seen as forbidding employers from denying women the right to get jobs that had been reserved for men.

By 1989, the Supreme Court, in Price Waterhouse v. Hopkins, upheld a sex discrimination claim brought by a woman who was denied partnership at an accounting firm because some male colleagues thought she was gruff and aggressive, not “feminine” in her attire.

Nearly a decade later, Justice Scalia spoke for a unanimous court in reviving a sex discrimination suit brought by a man who had worked on an oil rig off Louisiana and complained he was harassed, bullied and threatened with rape by the other men in showers. Scalia said this was clearly discrimination based on sex, even though it involved only men.

“Male-on-male sexual harassment was assuredly not the principal evil Congress was concerned with when it enacted” the 1964 civil rights law, Scalia wrote in Oncale v. Sundowner Offshore Services. “But statutory prohibitions often go beyond the principal evil to cover reasonably comparable evils,” and the words of the law can be easily read to forbid “sexual harassment of any kind,” he said.

The administration's lawyers cited the Hopkins and Oncale decisions in Friday's letter to school officials. Together, they say the decisions stand for the proposition that sex-related mistreatment is broadly prohibited by civil rights laws.

But Sen. Lamar Alexander, a former Tennessee governor and chairman of the Senate Education Committee, said, “This is the kind of issue that parents, schools boards, communities, students and teachers should be allowed to work out in a practical way. ... The guidance issued by the departments does not amount to federal law and should not be treated as such.”

Ed Whelan, a conservative legal analyst and a former Scalia law clerk, called Friday's announcement an “absurd misreading of Title IX.”

But Obama aides contend the directive is meant to clear up confusion and comes in response to a request from school principals seeking legal and practical guidance.

Tribune Newspapers' Christi Parsons contributed.

dsavage@tribune.com