WASHINGTON — A legal assault on public unions collapsed Tuesday when the Supreme Court deadlocked over a California woman's lawsuit to strike down mandatory fees, the strongest evidence yet that Justice Antonin Scalia's death has stymied the court's conservatives.

The 4-4 split keeps in place a 1970s-era rule that authorizes unions to require municipal employees, teachers, college instructors and transit workers to pay a “fair share fee” to help cover the cost of collective bargaining.

The tie vote, though expected after Scalia's death in February, came as a relief to union officials who feared the conservative justices were on the brink of striking down the pro-union law as a violation of free speech.

In another sign Tuesday that the high court continues to grapple with the vacancy left by Scalia, justices asked for additional briefings in a pending dispute over the Obama administration's contraceptive mandate under the Affordable Care Act.

Tie votes could be a theme this year as justices vote on several major disputes that divide along ideological lines, including abortion, election districts and immigration.

The White House said the court's deadlock in the union case underscores the need for the Senate to confirm the president's nominee, Merrick Garland, chief judge of the U.S. Court of Appeals for the District of Columbia Circuit, to replace Scalia.

The Republican-controlled Senate is refusing to act on Garland's nomination, saying the next president should fill the seat.

“With a Supreme Court that's not fully staffed, it makes it more likely that situations can arise across the country with different rulings in different courts that aren't resolved by the Supreme Court,” White House press secretary Josh Earnest told reporters aboard Air Force One.

Earlier this month, the court split in a narrow case involving spousal liability and gender discrimination, the first such vote since Scalia's death.

The deadlock in the union case leaves in place mandatory fees allowed by law in California and 22 other mostly Democratic states.

Such fees are prohibited in “right to work” states across the South and in much of the Midwest.

Orange County teacher Rebecca Friedrichs and several others had sued to overturn the fees, saying they objected to being forced to support the California Teachers Association.

The U.S. 9th Circuit Court of Appeals rejected her lawsuit, citing the 1977 Supreme Court ruling in Abood v. Detroit Board of Education, which had authorized these fair share fees in the first place.

That case held that workers could be required to share in the cost of collective bargaining, but they did not have pay for a union's political activities.

Before Scalia's death, the court's five more conservative justices had served notice that they were ready to overturn Abood and declare such forced fees as unconstitutional.

Instead Tuesday, the justices issued a one-line statement saying the 9th Circuit's ruling is “affirmed by an equally divided court.”

Labor law scholars said unions would have been crippled if nonmembers had no longer been required to pay anything to support the union.

“It would have been like a knife in the heart of the unions,” said Gary Chaison, professor of industrial relations at Clark University in Massachusetts.

The National Education Association, the nation's largest union with 3 million members, hailed the outcome as a victory.

Eric Heins, president of the California teachers group, said “wealthy corporate special interests” had brought the case to “make it harder for working families and the middle class to come together, speak up and get ahead. Now it's time for senators to do their job and appoint a successor justice to the highest court in our land.”

A labor policy expert who backed Friedrichs said challengers should now look to state legislatures to strike down the fees.

“With a divided court, thousands of public servants around the nation must still financially assist a government union that they disagree with,” said Trey Kovacs, a labor expert with the Competitive Enterprise Institute.

“Now it is up to state legislatures to provide public employees with the freedom to choose whether or not to pay for union representation,” Kovacs said.

dsavage@tribune.com