– Behind the lawsuits and all the rhetoric, the legal clash over transgender rights pitting North Carolina against the Justice Department boils down to a fundamental disagreement over a definition.

Pamela Karlan, who teaches public interest law at Stanford University, summed it up this way:

“The real question here is: How do we determine what a person’s sex is?”

The answer goes to the heart of whether federal civil rights laws that bar sex discrimination protect transgender Americans. On Friday, the Obama administration told all public school districts across the nation that they should allow transgender students to use the bathrooms that match their gender identity. That makes North Carolina ground zero in a cultural battle that has now gone nationwide.

It’s an issue the U.S. Supreme Court has yet to touch, and it may be the ultimate arbiter.

Dueling state and federal suits over North Carolina’s law barring transgender people from using men’s or women’s bathrooms or locker rooms in government buildings also underscore the political and cultural divide between many conservatives and liberals over issues surrounding sexual orientation.

Which side has the stronger case, given that lower courts have split on the issue, is a matter for debate.

The Republican-controlled General Assembly passed House Bill 2 in March in response to Charlotte’s extension of its anti-discrimination ordinance.

That ordinance would have allowed transgender people to use the bathroom of the gender with which they identify and would have extended anti-discrimination protections to LGBT people. HB2 pre-empted that ordinance, requiring people to use the bathrooms in government buildings that match what’s on their birth certificate.

Difference in definitions

The Obama Justice Department contends that sexual orientation is not a personal decision, but is based on an array of factors affecting an individual’s sense of gender and is protected by civil rights laws enacted 40 and 50 years ago. In slapping the state of North Carolina with a suit charging HB2 violates sex discrimination provisions in the Civil Rights Act of 1964, a 1972 education law and the 2013 Violence Against Women Act, Attorney General Loretta Lynch accused North Carolina officials of “state-sponsored discrimination against transgender individuals.”

North Carolina Gov. Pat McCrory and legislative leaders filed separate suits asking that a judge determine the law is not discriminatory.

The 1964 law barring employment discrimination on the basis of “race, color, religion, sex and national origin” says it’s illegal to “discriminate against people based on their sex,” Karlan, who was a senior official of the Civil Rights Division in 2014 and 2015, said. “It doesn’t say you can’t discriminate against people based on the sex on their birth certificate.”

But Ed Whelan, president of the conservative-leaning Ethics and Public Policy Center and head of its program on the Constitution, the Courts and the Culture, says laws like North Carolina’s “do not involve by any conceivable measure discrimination on the basis of gender identity.”

North Carolina’s law disregards gender identity by defining gender as a person’s biological sex at birth, Whelan, a former law clerk to the late conservative Supreme Court Justice Antonin Scalia, said.

McCrory’s lawsuit notes that in 2000, the 7th U.S. Circuit Court of Appeals held that Congress “intended the term ‘sex’ to mean ‘biological male or biological female,’ and not one’s sexuality or sexual orientation.”

In 1982, the 8th Circuit found that, because Congress had failed to make transgender people a protected class, discrimination based on their gender identities is not covered by Title VII of the 1964 Civil Rights Act.

Bathroom blunders

Proponents of both sides say cringe-worthy bathroom scenarios will occur if they lose the argument.

Karlan said that she would be deeply “disconcerted” about what might happen if transgender people were forced to use the bathrooms of the sex they abandoned.

For example, she said, “what if somebody wearing a mustache and a beard with a suit and tie comes into the [women’s] bathroom and says, ‘Actually, if you saw the genitalia I was born with, you wouldn’t be upset.’ ”

Backers of HB2 say, however, that ordinances such as Charlotte’s create a safety risk because men dressed as women would share bathrooms with women and young girls.

Whelan said the Obama administration “started this war by insisting there has to be a one-size-fits-all national policy on this. They’re the ones who are insisting that a guy who looks like a guy but feels like a woman has to use the women’s room.”

If the North Carolina law were upheld, transgender people would have to use unisex or other alternative bathrooms — an outcome that Attorney General Lynch called hurtful and discriminatory.

‘No clear identification’

McCrory told Fox News that Obama administration officials are “trying to define gender identity. And there is no clear identification or definition of gender identity.”

Perhaps, but Karlan said the medical profession has recognized that “people’s sex is made up of a complex group of factors, and for most people, the factors tend to reinforce each other.”

For most people, she said, “their chromosomal sex and their external genitalia and their sense of a social role, and the hormones that are coursing through their bodies all kind of align. For transgender people, their chromosomal sex and external genitalia at birth don’t necessarily correlate with their internal sense of gender.”

Lynch told a news conference that “it is our position that federal law has been clear for some time now that discrimination against sex includes discrimination against individuals based on sexual identity and gender identity.”