WASHINGTON — The Supreme Court on Monday struck down a Louisiana law that could have left the state with a single abortion clinic, dashing the hopes of conservatives who were counting on President Trump’s appointments to lead the court to sustain restrictions on abortion rights and, eventually, to overrule Roe v. Wade.

Instead, conservatives suffered a setback, and from an unlikely source. Chief Justice John G. Roberts Jr. added his crucial fifth vote to those of the court’s four-member liberal wing, saying that respect for precedent compelled him to do so, even though he had voted to uphold an essentially identical Texas law in a 2016 dissent.

In the past two weeks, the chief justice has voted with the court’s liberal wing in three major cases: on job discrimination against lesbian, gay, bisexual and transgender workers, on a program protecting young undocumented immigrants known as Dreamers and now on abortion. While he has on occasion disappointed his usual conservative allies, notably on the Affordable Care Act and adding a citizenship question to the census, nothing in his 15-year tenure on the court compares to the recent run of liberal votes in major cases.

Conservatives reacted with fury. “Chief Justice Roberts is at it again with his political gamesmanship,” Senator Ted Cruz, Republican of Texas, said on Twitter. “This time he has sided with abortion extremists who care more about providing abortion-on-demand than protecting women’s health.”

Progressive groups countered that the court’s decision was a routine application of precedent.

“Today’s ruling, while incredibly important, should not be surprising,” Elizabeth Wydra, the president of the Constitutional Accountability Center, a liberal group, said in a statement. “In fact, this case should have been one of the easiest for the court to dispose of this term.”

“This was really just ‘Supreme Court 101’ for all nine of the justices,” she said, “but only five of them were prepared to show fidelity to law and precedent instead of politics.”

Justice Stephen G. Breyer, writing for the four liberals who joined with the chief justice in the majority, said the Louisiana law was “almost word-for-word identical” to the one from Texas that the Supreme Court struck down in the 2016 decision, Whole Woman’s Health v. Hellerstedt.

Both laws required doctors performing abortions to have admitting privileges at nearby hospitals. And in both cases, Justice Breyer wrote, the laws put an undue burden on the constitutional right to the procedure.

The court’s decision to revisit the issue of admissions privileges had worried proponents of abortion rights given Chief Justice Roberts’s support for the Texas law. Since that ruling, Justice Anthony M. Kennedy, who had voted to overturn the law, was replaced by the more conservative Justice Brett M. Kavanaugh.

But in the end, Chief Justice Roberts’s commitment to precedent sank the Louisiana law. “I joined the dissent in Whole Woman’s Health,” he wrote on Monday, “and continue to believe that the case was wrongly decided. The question today, however, is not whether Whole Woman’s Health was right or wrong, but whether to adhere to it in deciding the present case.”

“The Louisiana law imposes a burden on access to abortion just as severe as that imposed by the Texas law, for the same reasons,” the chief justice wrote in a concurring opinion that did not adopt Justice Breyer’s reasoning. “Therefore Louisiana’s law cannot stand under our precedents.”

Still, it would be hasty to conclude that Chief Justice Roberts was prepared to strike down other abortion restrictions or that he would vote to sustain the Roe decision, which in 1973 established a constitutional right to abortion, should a direct challenge to the ruling reach the court.

Kayleigh McEnany, the White House press secretary, expressed disappointment in Monday’s decision.

“In an unfortunate ruling today,” she said in a statement, “the Supreme Court devalued both the health of mothers and the lives of unborn children by gutting Louisiana’s policy that required all abortion procedures be performed by individuals with admitting privileges at a nearby hospital.”

Joseph R. Biden Jr., the presumptive Democratic presidential nominee, issued a statement saying that “women’s health care rights have been under attack as states across the country have passed extreme laws restricting women’s constitutional right to choice under any circumstance.”

Justice Breyer wrote that the Louisiana law, which was enacted in 2014, imposed great burdens on access to abortion but did nothing to protect women’s health, its ostensible goal. He wrote that hospitalizations after abortions were rare, that women would receive medical care at hospitals whether their doctors had admitting privileges or not and that abortion providers were often unable to obtain admitting privileges for reasons unrelated to their competence.

Only two of the five doctors who provide abortions in Louisiana have obtained admitting privileges, one in New Orleans and one in Shreveport. But the Shreveport doctor testified that he could not handle the clinic’s work alone. If the law went into effect, a trial judge concluded, there would be a single doctor in a single clinic, in New Orleans, available to provide abortions in Louisiana.

The evidence in the Louisiana case, Justice Breyer wrote, was “even stronger and more detailed” than in the Texas case.

The vote in the Texas decision was 5 to 3, with Justice Kennedy joining the court’s four-member liberal wing to form a majority. It was decided by an eight-member court after the death of Justice Antonin Scalia, in 2016. Mr. Trump appointed Justice Neil M. Gorsuch to succeed Justice Scalia the next year.

On Monday, Justice Breyer wrote that the Louisiana law would severely restrict abortion as a practical matter.

“A Shreveport resident seeking an abortion who might previously have obtained care at one of that city’s local clinics would either have to spend nearly 20 hours driving back and forth,” Justice Breyer wrote, “or else find overnight lodging in New Orleans,” as the state imposes a waiting period between an initial consultation and the procedure. He added, “The burdens of this increased travel would fall disproportionately on poor women, who are least able to absorb them.”

Justice Breyer concluded that the law posed a “substantial obstacle” to women seeking abortions and placed an “undue burden” on their constitutional rights, drawing on tests established in the court’s 1992 ruling in Planned Parenthood v. Casey.

Justices Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan joined Justice Breyer’s plurality opinion.

In his concurrence, Chief Justice Roberts questioned whether that opinion had imposed an amorphous balancing test not warranted by the court’s precedents. But he said the test announced in the Casey decision was enough to decide the case.

“I agree with the plurality that the determination in Whole Woman’s Health that Texas’s law imposed a substantial obstacle requires the same determination about Louisiana’s law,” he wrote. “Under those same principles, I would adhere to the holding of Casey, requiring a substantial obstacle before striking down an abortion regulation.”

In dissent, Justice Samuel A. Alito Jr., joined by Justices Gorsuch, Kavanaugh and Clarence Thomas, wrote that the Louisiana law protected the health and safety of women seeking abortions and that the requirements for obtaining admitting privileges helped ensure the competence of doctors. The facts on the ground in the two states, he wrote, were enough to require a different conclusion.

“There is ample evidence in the record showing that admitting privileges help to protect the health of women by ensuring that physicians who perform abortions meet a higher standard of competence than is shown by the mere possession of a license to practice,” Justice Alito wrote. “In deciding whether to grant admitting privileges, hospitals typically undertake a rigorous investigative process to ensure that a doctor is responsible and competent and has the training and experience needed to perform the procedures for which the privileges are sought.”

Chief Justice Roberts disagreed. “Appreciating that others may in good faith disagree,” he wrote, “I cannot view the record here as in any pertinent respect sufficiently different from that in Whole Woman’s Health to warrant a different outcome.”