More than 40 people set up camp outside the Supreme Court the day before it heard a case about Texas’s abortion clinic regulations. They waited in line through the pouring rain for a chance to sit in the courtroom. (Jenny Starrs/The Washington Post)

The Supreme Court’s liberal justices united Wednesday to attack Texas’s abortion regulations as an unconstitutional burden on a woman’s rights, but the justice who holds the key vote left the court’s ultimate resolution of the issue in doubt.

Justice Anthony M. Kennedy, whom both sides consider pivotal to the outcome of the court’s most important abortion case in a generation, wondered whether there was enough evidence to decide that changes in Texas law were responsible for the closure of nearly half of the abortion clinics in the nation’s second-largest state. He wondered whether lower courts might need to do more work on the issue.

But Kennedy also questioned the law in a way that would give hope to those who support abortion rights. He was reluctant to accept Texas’s arguments that there was no reason to weigh the state’s grounds for making the 2013 changes against the obstacles created for women.

And he seemed concerned that a lack of access might be leading to women undergoing abortions later in their pregnancies, which are riskier. “This may not be medically wise,” Kennedy said.

If Kennedy joins the liberals to make a five-member majority, it will have national implications, cutting off what abortion opponents had seen as a promising way to make abortion more rare. Abortion rights backers say more than 200 restrictions have been passed by states in the past five years.

South Texas's only abortion clinic, located in the border town of McAllen, has become a battleground for abortion activists on both sides. (Whitney Leaming/The Washington Post)

The case is being heard by only eight members after the death last month of Justice Antonin Scalia, a conservative who did not believe that the Constitution protected the right to abortion.

If Kennedy sides with the three remaining conservatives, that will not be enough by itself to secure the court’s endorsement of the two issues at stake: requiring admitting privileges at a nearby hospital for doctors who perform abortions and requiring clinics to maintain hospital-like standards.

A number of states have adopted such standards, which they say improve patient care. But abortion providers say the rules are medically unnecessary and so expensive or hard to satisfy that they force clinics to close.

While a 4-to-4 tie would uphold a lower court’s decision approving the Texas law, it would not set a national precedent. The restrictions have been found unconstitutional in other courts across the nation.

The absence of Scalia affected the arguments in other ways. He was a dominating questioner, and when he and Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr., fellow conservatives, worked in concert, they formed an intimidating bloc.

On Wednesday, it was the court’s four liberals who dominated the questioning.

“What it’s about is that a woman has a fundamental right to make this choice for herself,” said Justice Ruth Bader Ginsburg. She and Justice Sonia Sotomayor questioned the lawyers for so long after Roberts had indicated time was up that the scheduled one-hour argument stretched for almost 30 more minutes.

Here is a look at abortion regulations around the country.

Stephanie Toti of the Center for Reproductive Rights, representing the clinics, told the court that Texas’s regulations violated the standard set in the court’s last majority abortion decision, 1992’s Planned Parenthood v. Casey.

That decision, written partly by Kennedy, balanced “states’ legitimate interests in regulating abortion and women’s fundamental liberty to make personal decisions about their pregnancies,” she said, adding that the Texas rules “are unnecessary health regulations that create substantial obstacles to abortion access.”

She contended that partial implementation of the law has meant the closure of about half of Texas’s more than 40 abortion clinics and that full implementation would reduce that to 10 or fewer in a state with more than 5 million women of reproductive age.

Alito questioned whether Toti had proved to lower courts that the new laws were the reason.

“There is very little specific evidence in the record in this case with respect to why any particular clinic closed,” Alito said.

Kennedy joined in that questioning, and that was when he wondered whether lower courts should take a closer look.

But Justice Elena Kagan said the facts of what has happened as the case has worked its way through the legal process answered the questions.

“It’s almost like the perfect controlled experiment as to the effect of the law, isn’t it?” she said. “It’s like you put the law into effect, 12 clinics closed. You take the law out of effect, they reopen.”

Roberts questioned Toti about whether the court should pass judgment on Texas’s grounds for passing the 2013 law, as long as the state had a rational reason for the changes.

“I thought the undue burden and substantial obstacle [test] went to whether it was undue in light of the woman’s right to exercise her right to an abortion,” Roberts said — either the restrictions went too far or they didn’t, regardless of the state’s intent.

With Justice Clarence Thomas asking no questions, the conservatives’ questioning was muted. At times they debated a technical legal point that, because of previous legal wrangling in the case, the clinics may have lost their right to a blanket ruling that the Texas law is unconstitutional.

U.S. Solicitor General Donald B. Verrilli Jr., representing the Obama administration, was forceful in his turn before the court.

“This law closes most abortion facilities in the state, puts extreme stress on the few facilities that remain open, and exponentially increases the obstacles confronting women who seek abortions in the state,” Verrilli said. “And it does all of that on the basis of a medical justification that cannot withstand any meaningful scrutiny.”

Verrilli seemed to score points with Kennedy when he said the remaining clinics that met the ambulatory surgical requirements — already required in Texas for second-trimester abortions — could not meet the demand.

There are about 65,000 to 70,000 abortions in Texas each year, Verrilli said, and clinics that meet the standards perform about 14,000 per year.

“Twenty percent,” Kennedy said.

Verrilli’s counterpart, Texas Solicitor General Scott A. Keller, said that Texas was meeting its constitutional obligations.

“Abortion is legal and accessible in Texas,” Keller said. “All the Texas metropolitan areas that have abortion clinics today will have open clinics if the court affirms, and that includes the six most populous areas of Texas.”

But he was immediately set upon by the liberal justices.

Kagan said that 750,000 women would live more than 200 miles away from an abortion clinic; before the law was passed, that number was 10,000.

Sotomayor questioned why a woman undergoing a medication abortion, which requires taking pills, needed to do so in a center equipped for surgery. “Your brief seemed to be telling us that there’s no role for the court to judge whether there’s really a health benefit to what you’re doing,” she said.

Ginsburg and Justice Stephen G. Breyer questioned the requirement that doctors have admitting privileges at hospitals within 30 miles of a center. For one thing, Ginsburg said, any complications happen after a woman has left the center and has returned home.

And Breyer challenged Keller for evidence of a single incident in which a woman was turned away from a hospital because the doctor performing an abortion lacked the required credential.

Keller conceded there was nothing in the record to indicate that.

The liberal justices said that liposuction and colonoscopies had higher rates of complications than first-trimester abortions but that those procedures were performed in doctor’s offices.

Keller said abortion could be treated differently under the court’s precedents and said legislators “react to topics that are of public concern.”

Kagan questioned the motives of the lawmakers. Even if Texas were allowed to set much higher medical standards for abortion clinics, she said, “I guess I just want to know why would Texas do that?”

Her point seemed to be that it was not to protect health but to close clinics.

The justices are applying a test the court formulated nearly 25 years ago in the Casey decision. It said states had a legitimate interest in regulating abortion procedures but could not make them so onerous as to impose an “undue burden” on a woman’s right to terminate a pregnancy before fetal viability.

Included in the description of such a burden was “unnecessary health regulations that have the purpose or effect of presenting a substantial obstacle to a woman seeking an abortion.”

While Texas presented medical experts who said the restrictions protected women, the clinics were supported by the American College of Obstetricians and Gynecologists and the American Medical Association, among other medical groups.

“Laws that regulate abortion should be evidence-based and ­designed to improve women’s health,” they said in a brief. “The challenged provisions of H.B. 2 are neither.”

A trial judge found the arguments unconvincing and struck down those provisions of the law. But a panel of the U.S. Court of Appeals for the 5th Circuit in New Orleans reversed that decision and said courts do not have to scrutinize the rationale provided by a legislature for abortion laws as long as that rationale has a reasonable basis.

The case is Whole Woman’s Health v. Hellerstedt.

Sandhya Somashekhar contributed to this report.