Chief Justice John Roberts Jr. emerged Wednesday as the pivotal, if inscrutable, key to whether a more conservative Supreme Court is ready to reconsider its precedents protecting abortion rights.

The court's four liberals appeared convinced that the Louisiana law at issue, requiring admitting privileges at nearby hospitals for abortion-clinic doctors, is identical to one from Texas that the Supreme Court struck down four years ago.

That appeared to leave Roberts, now at the court's ideological centre, holding the deciding vote.

As the case was argued, he asked a variation of the same question to all three lawyers addressing the justices: Should the court undertake a fact-dependent, state-by-state analysis when reviewing such restrictions?

That could indicate he felt he could distinguish Louisiana's law from the Texas statute. Or it could mean he agreed with challengers that similar laws were doomed because of the court's precedent from 2016. Roberts, a dissenter in the Texas case, did not indicate what he believed was the proper answer.

The justices' examination of the Louisiana law could be momentous if it signalled that the court was ready to revisit past decisions. And whatever it decides before the end of June will come in the midst of a heated presidential election in which Democrats have made abortion rights a priority and President Donald Trump has said his ultimate goal is to overturn Roe v Wade.

Trump has said his judicial nominees will oppose abortion, and it was the first major abortion case for his Supreme Court choices, Neil Gorsuch and Brett Kavanaugh.

Gorsuch did not ask a question during the hour-long oral arguments. Kavanaugh's questioning was similar to that of Roberts, as he wondered whether different conditions in states could afford different outcomes than in the 2016 case, Whole Women's Health v Hellerstadt.

"Are you saying admitting-privileges requirements are always unconstitutional, such that we don't have to look at the facts state by state?" Kavanaugh asked attorney Julie Rikelman after posing a hypothetical about a state with 10 clinics and 20 doctors, all with the necessary credentials. Kavanaugh had previously voted to allow the Louisiana law to go into effect, saying it was unclear whether doctors had worked hard enough to secure the privileges.

Rikelman, senior litigation director of the Centre for Reproductive Rights, replied that "the burdens may vary, but a law that has no benefit and serves no valid state interest, which is what this court held in Whole Woman's Health, is much more likely to be an undue burden" on a woman's right to abortion.

Kavanaugh pressed: "Could an admitting-privileges law of this kind ever have a valid purpose, in your view?"

"No, your honor," Rikelman replied. "The medical consensus against these laws is clear." Later, she noted that the federal government has dropped such a requirement from its regulations of surgery centers nationwide.

If the debate inside the courtroom was reminiscent of four years ago, the atmosphere inside and outside the court was emblematic of the decades that the Supreme Court has wrestled with one of the country's most divisive issues.

Members of Congress packed the crowded courtroom, and lawyers who have litigated the issue from both sides filled the pews. Among Wednesday's spectators was Roberts' wife, Jane Roberts, who before the chief justice joined the court was legal counsel for Feminists for Life of America, a group that opposes abortion.

At a rally of abortion rights supporters outside the court, Senate Majority Leader Charles Schumer, D-N.Y., called out Gorsuch and Kavanaugh by name.

"You have released the whirlwind, and you will by the price," Schumer said. "You won't know what hit you if you go forward with these awful decisions."

Schumer's remarks prompted a rare public rebuke from Chief Justice Roberts, who issued a statement hours later calling the criticism from the highest levels of government in the midst of arguments "inappropriate" and "dangerous."

Inside the courtroom, Justice Stephen Breyer acknowledged the contentious public divide.

"In the country, people have very strong feelings and a lot of people morally think it's wrong and a lot of people morally think the opposite is wrong," said Breyer, adding, "I think personally the court is struggling with the problem of what kind of rule of law do you have in a country that contains both sorts of people."

The justices were considering whether Louisiana's 2014 law unduly burdens women's access to abortion. Practitioners claim, and a federal judge agreed, that the law could force two of the state's abortion clinics to close, leaving only one doctor at one clinic to perform the procedures.

The question of whether the abortion providers had legal standing to bring the case was not in the 2016 case. But Justice Samuel Alito Jr. sounded highly skeptical Wednesday that clinics have legal grounds to sue, calling such a claim "amazing" because of the doctors' potential conflict of interest.

To reject the clinic's right to sue, Rikelman said, would require the court to overrule at least eight past abortion-related decisions.

Liberals said the court would be reversing decades of precedent to make such a finding, and the court's other conservatives did not ask about the issue.

The court could reaffirm or overturn its 2016 precedent in the Texas case, or distinguish it in a way that a restriction deemed unconstitutional in one state is allowed in another.

One change at the court: While the Justice Department in 2016 urged the justices to strike down the Texas law, Trump administration lawyers argued that Louisiana's should be left in place, even if it means overturning the opinion in the Texas case.

Jeffrey Wall, principal deputy solicitor general, representing the Trump administration, urged the court to find that medical providers do not have a legal right to challenge the Louisiana regulations on behalf of their patients because their respective interests are not necessarily aligned.

"One is the interest of the for-profit providers and not being regulated in particular ways. The other is the interest of women in their own health and safety," he said.

Wall noted that a patient who developed a complication once at home after an abortion might still call the clinic to consult with her initial doctor before going to the hospital.

"It's hard for me to believe that women in Louisiana wouldn't at least want the option to be treated by the [same] doctor," he said.

Hospitalisation after an abortion is rare, all sides agree, and the lack of admitting privileges by the doctor who performed the procedure is not a bar to the woman getting needed medical care. In the court's Texas decision, the majority said there are numerous reasons doctors might not be able to attain admitting privileges at a nearby hospital, including the fact that it is so rare for their clients to need hospitalisation.

The court majority in that case also said the admitting-privileges requirement "provides few, if any, health benefits for women, poses a substantial obstacle to women seeking abortions, and constitutes an 'undue burden' on their constitutional right to do so."

Justice Ruth Bader Ginsburg stressed Wednesday that most women who terminate their pregnancies never get near a hospital. About 40 percent of abortions in Louisiana are completed by taking medication.

Elizabeth Murrill, Louisiana's solicitor general, said the state's 2014 law is intended to ensure the health and safety of women seeking abortions and distinct from the Texas law. Regulations for abortion providers, she said, are consistent with those for the state's surgery centres - both require medical staff to have admitting privileges.

"Even if it's a medication abortion, the doctor should be able to handle a surgical abortion and be qualified to do that," Murrill said, asking the court to affirm a ruling from the U.S. Court of Appeals for the 5th Circuit.

A Louisiana federal judge held a six-day trial on the law and agreed with abortion providers that the court's decision in Hellerstedt meant that the law was unconstitutional.

But a panel of the 5th Circuit - the same court that had upheld the Texas law - disagreed. The panel, on a 2-to-1 vote, found factual distinctions between how the restriction played out in Texas and Louisiana.

Judge Jerry Smith, writing for the majority, said that the court complied with the Supreme Court's decision by taking a painstakingly close look at the details.

"Unlike in Texas, the [Louisiana law] does not impose a substantial burden on a large fraction of women," he concluded.

The full 5th Circuit declined to reconsider the panel's decision, and dissenting judges said their colleagues seemed more intent on giving the Supreme Court a chance to reverse its 2016 ruling than complying with it.

"I am unconvinced that any justice of the Supreme Court who decided Whole Woman's Health would endorse our opinion," Judge Stephen Higginson wrote in his dissent. "The majority would not, and I respectfully suggest that the dissenters might not either."

The Louisiana case is June Medical Services v Russo.