WASHINGTON — A divided Supreme Court on Monday struck down a Louisiana law regulating abortion clinics, reasserting a commitment to abortion rights over fierce opposition from dissenting conservative justices in the first big abortion case of the Trump era.

Chief Justice John Roberts and his four more liberal colleagues ruled that the law requiring doctors who perform abortions to have admitting privileges at nearby hospitals violates abortion rights the court first announced in the landmark Roe v. Wade decision in 1973.

The outcome is not the last word on the decades-long fight over abortion with dozens of state-imposed restrictions winding their way through the courts. But the decision was a surprising defeat for abortion opponents, who thought that a new conservative majority with two of President Donald Trump’s appointees on board would start chipping away at abortion access.

The key vote belonged to Roberts, who had always voted against abortion rights before, including in a 2016 case in which the court struck down a Texas law that was virtually identical to the one in Louisiana.

The chief justice explained that he continues to think the Texas case was wrongly decided, but believes it’s important for the court to stand by its prior decisions.

“The result in this case is controlled by our decision four years ago invalidating a nearly identical Texas law,” Roberts wrote. He did not join the opinion written by Justice Stephen Breyer for the other liberals in Monday’s decision, and his position left abortion-rights supporters more relieved than elated.

The case was the third in two weeks in which Roberts, a George W. Bush appointee, joined the court’s liberals in the majority. One of the earlier decisions preserved the legal protections and work authorization for 650,000 immigrants who were brought to the U.S. as children. The other extended federal employment-discrimination protections to LGBT Americans, a decision that Justice Neil Gorsuch also joined and wrote.

In dissent on Monday, Justice Clarence Thomas wrote, “Today a majority of the Court perpetuates its ill-founded abortion jurisprudence by enjoining a perfectly legitimate state law and doing so without jurisdiction.”