(Reuters) - During arguments at the U.S. Supreme Court in this week's major transgender rights case, liberal Justice Sonia Sotomayor told the lawyer defending Tennessee's ban on gender-affirming medical care for people under age 18 that courts have a historic role in protecting minorities from discrimination. 

"When you're 1 percent of the population or less, (it is) very hard to see how the democratic process is going to protect you," Sotomayor told Tennessee Solicitor General Matthew Rice during Wednesday's arguments, referring to the transgender population. 

Sotomayor's observation underscored key legal issues in the dispute - the role of U.S. courts to serve as a check on the legality of actions by legislatures and the level of scrutiny they should apply to laws under legal challenge. 

The Supreme Court, which has a 6-3 conservative majority, appeared ready to uphold Tennessee's Republican-backed law, which was challenged by President Joe Biden's administration and other plaintiffs but was affirmed by a lower court.

The administration wants the justices to conclude that the ban represents unlawful discrimination based on sex, which under Supreme Court precedent would trigger tougher judicial review and make is harder to defend under the U.S. Constitution's 14th Amendment guarantee of equal protection. 

But the lower court rejected that approach. Instead, it used what is called a rational-basis review merely requiring a rational connection between a law and a legitimate state interest. 

A ruling by the Supreme Court using the rational-basis approach to uphold Tennessee's law, according to legal experts, could make it easier to defend a broader array of measures targeting transgender people, from bathroom use to sports participation, and extend even to restrictions on adults.

The Tennessee law bans medical treatments such as puberty blockers and hormones for adolescents experiencing gender dysphoria, the clinical diagnosis for significant distress that can result from an incongruence between a person's gender identity and the sex assigned at birth.

Some of the conservative justices emphasized an ongoing debate among experts and policy makers over the potential benefits and risks of the treatments, suggesting the need for deference to elected legislators to make policy. 

"If the court holds that discrimination against transgender people does not constitute sex discrimination and does not otherwise qualify for heightened scrutiny under the equal protection clause, then equal protection challenges to other forms of anti-trans governmental discrimination will be tough to win," said Paul Smith, a Georgetown University law professor who has argued many cases at the Supreme Court including a landmark gay rights victory in 2003.

Conservative Justice Brett Kavanaugh noted that certain European countries that he said have been at the forefront of youth transgender care policy are "pumping the brakes" on these treatments. In light of this, Kavanaugh signaled hesitance about having the Supreme Court "constitutionalize the whole area."

Liberal justices expressed alarm that the court would not scrutinize legislative choices to ensure that they are not merely aimed at harming a disfavored minority group. 

"What's really going on here is discrimination against - a disregard for - young people who are trans," liberal Justice Elena Kagan said. 

Some legal experts said wider implications of favoring Tennessee in the case would depend on the rationale expressed in the eventual ruling, expected by the end of June. 

Tennessee has argued that the ban is based on the treatment's medical purpose, not the patient's sex. 

If the court decides that laws making "biologically rooted distinctions" between individuals are not subject to tougher judicial review, for instance, "then there might still be heightened scrutiny for distinctions that are more distant from biology as such, like bathrooms and sports," said University of Mississippi School of Law professor Christopher Green, who filed a legal brief backing Tennessee. 

This law is among numerous measures pursued in recent years, primarily in Republican-led states, concerning LGBT rights including limits on discussions of sexual orientation or gender identity in public schools, reinforcing sex-separated spaces like bathrooms in public places, and participation by transgender athletes in women's sports.

'SERIOUS HARM'

Though Tennessee's law targets transgender adolescents, a ruling upholding it could make it easier for legislatures to impose restrictions on transgender adults as well.

"If the court finds that there is no sex classification involved in (the ban), it seems hard to imagine they would find one in a ban on gender-affirming medical care for adults," said lawyer Karen Loewy of the LGBT rights group Lambda Legal, which helped represent transgender minors and their parents who challenged the law. 

Upholding the lower court's ruling could "do serious harm to decades of sex-discrimination law, which has required heightened scrutiny for any law rooted in overbroad generalizations about men and women," Loewy said.

On Wednesday, U.S. Solicitor General Elizabeth Prelogar, arguing for Biden's administration, urged the justices to adopt the tougher judicial review. 

"Maybe the states will ban medical care for adults who are transgender, maybe they'll ban adoption by transgender people or not allow them to be teachers," Prelogar said. "That doesn't look anything like the workaday economic regulation that just gets rational-basis review."

Rice said that to the extent that a law dealing with adults would pass rational-basis review, "that just means it's left to the democratic process - and that democracy is the best check on potentially misguided laws." 

The administration of President-elect Donald Trump, who as a candidate advocate restricting transgender rights, could reverse the position taken by Biden's administration. Were that to occur, the court still might decide the case given that other challengers to the law also argued on Wednesday.