The Supreme Court confronts a major civil rights issue Tuesday over how broadly the justices should read the word “sex” in a 55-year-old anti-discrimination law — and a key aspect is Congress’ current push to clarify that the law covers LGBT individuals.

Title VII of the Civil Rights Act of 1964 prohibits private companies from discriminating against employees on the basis of “sex,” seen at the time as a historic step for women’s rights.

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But in oral argument on a trio of cases, the justices will delve into whether that word — sex — means the law also gives those protections to gay, lesbian and transgender workers.

Democratic lawmakers, who this year advanced legislation to ensure LGBT workplace protections under Title VII no matter how the court rules, told the justices in a brief that “sex” already covers LGBT workers. Republican lawmakers filed a brief in the case to argue that it doesn’t.

Beyond those competing interpretations, the justices will grapple with whether they should broaden a law beyond its apparent original intent, particularly when Congress has debated the policy for years and never passed it.

The cases will be the first time the Supreme Court will decide an LGBT rights case since the retirement of Justice Anthony M. Kennedy, who wrote a series of opinions such as the legalization of same-sex marriage in 2015 that helped usher in a new era of civil rights.

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And it could have sweeping social implications, since 28 states have no express protections for LGBT employee rights, said Ria Tabacco Mar, a senior staff attorney with the ACLU’s LGBT and HIV Project who has worked on two of the cases.

“We’re talking about the ability to earn a living, the ability to support our families, the ability to secure a safe place to live. This goes to the very heart of what it is to live and work in this country,” Tabacco Mar said.

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Lower court judges who decided the three cases now at the Supreme Court — from a transgender funeral home worker, a gay skydiving instructor and a gay county worker — did not necessarily follow their usual ideological tilt, legal experts said.

The four justices who make up the Supreme Court’s liberal wing are expected to interpret the word “sex” more broadly to include sexual orientation and gender identity. The question is whether a conservative justice will join them or say the policy decision is more suited to Congress.

Among those conservative justices, particularly Chief Justice John G. Roberts Jr., there will be a powerful sense that “if Congress wants to do this, Congress should speak clearly, and we’re going to put the ball back into Congress’ court,” Kannon Shanmugam, veteran Supreme Court advocate at law firm Paul Weiss, said during a panel discussion.

Jonathan Turley, a professor at George Washington Law, said the five conservative justices will be uncomfortable with too much of a departure from the original intent and moving the court ahead of Congress on the issue.

The House in May passed its bill, 236-173, to prohibit discrimination under Title VII based on sexual orientation and gender identity. Eight Republicans joined all 228 Democrats on that vote. It is likely to stall in the Republican-controlled Senate.

“You have Congress trying to pass a bill as we speak; the justices will be aware of that,” Turley said.

A group of eight Republican senators and 33 Republican representatives filed a brief in the case to stress that point. Congress has expressly included sexual orientation as a protected class in other laws, the Republicans wrote, and Title VII does not.

“The legislature has the ability to modify Title VII. If Congress intended to include sexual orientation and gender identity among the protected classes in Title VII, it could have done so,” the Republican brief states.

A group of 39 Democratic senators and 114 Democratic representatives, including Speaker Nancy Pelosi of California and bill author Rep. David Cicilline of Rhode Island, filed a brief that argues the plain text of Title VII covers sexual orientation and gender identity even if bills to clarify that have not passed.

“And because Congress’s intent in passing Title VII was to prevent invidious workplace discrimination, the statute is entitled to a broad construction and should not be read in an artificially restrictive way that would leave many Americans unprotected from sex-based discrimination,” the lawmakers wrote.

The Democratic lawmakers point to the bill, called the Equality Act, and the accompanying House report, to show that it was “designed to make explicit” that sexual orientation and gender identity claims are covered under Title VII.

And they quote previous Supreme Court rulings that have expanded the meaning of Title VII, such as a 1989 ruling that found the law was intended to strike at “the entire spectrum of disparate treatment of men and women resulting from sex stereotypes.”

As the House considered the measure, Pelosi said on the floor that the bill is the latest in a history of congressional action to expand LGBT rights, and House Democrats “fought the fight to present the case in the court of public opinion and to bolster the case in the Supreme Court.”

The Obama administration interpreted Title VII in the same way as Democratic lawmakers through the Equal Employment Opportunity Commission. But the Trump administration flipped positions, and cites congressional inaction to make their case.

The Justice Department argued in a brief that Congress did not add those protections when it overhauled Title VII in 1991, and until 2017 every appeals court agreed that Title VII doesn’t prohibit sexual-orientation discrimination.

“Congress’s consistent refusal to change that aspect of Title VII in the face of that uniform understanding, while at the same time changing other aspects of Title VII and expressly barring sexual-orientation discrimination in other statutes, supports adhering to that understanding now,” the DOJ wrote.

The justices will decide the case by the end of the term at the end of June.