When Mississippi’s mostly white and Republican-controlled State Legislature voted last spring to add a state-run police force and Court system atop the existing ones in the state’s mostly Black, Democratic-run capital, Jackson, some residents went to court to fight what they called an assault on their right to self-government.

Now the State Supreme Court has handed them a small victory, ruling unanimously that the Legislature had no authority to add four state-appointed judges to the locally elected Circuit Court that handles most of the city’s court cases.

The addition of four “special circuit judges” to the Hinds County Circuit Court was one aspect of much broader legislation that lawmakers called an attempt to address violent crime in Jackson, a city of 150,000.

Other parts of the measure expanded the jurisdiction of a small state police force, originally set up to guard government buildings, to encompass the entire city and established a new, separate court system to serve an area encompassing most of the city’s predominantly white neighborhoods.

In its ruling, the court said the State Constitution requires that circuit court judges be elected in all but extraordinary circumstances, such as the replacement of a judge who is disabled or disqualified. The legislation cited “nothing special or unique” that justified adding unelected judges to the court, the ruling stated.

Elsewhere, the ruling said the Constitution gave the Legislature authority to set up a separate court system in Jackson. That action, and much of the rest of the legislation, is being challenged in a separate lawsuit in United States District Court for the Southern District of Mississippi.

Cliff Jackson, who directs the MacArthur Justice Center at the University of Mississippi and who argued the case before the State Supreme Court, said the ruling closed an avenue that the Legislature could have used to place judges in other predominantly Black cities in the state. But beyond that, he said, it restored a measure of local control to “an 83 percent Black city that has four Black circuit judges elected by local folks.”

“The fact that they were able to beat this back and get a victory in the context of this effort to control Jackson means an awful lot,” he said.

The federal suit challenging the Legislature’s action, brought by the N.A.A.C.P., has yet to come to trial. It seeks to dismantle virtually all of the expanded police and court systems ordered in Jackson, arguing that the law authorizing them is an intentional act of racial discrimination that violates the equal protection guarantee of the 14th Amendment.

The suit took on new significance in July when the Civil Rights Division of the federal Justice Department joined the N.A.A.C.P. as a plaintiff. The department’s brief in the case argues that the law “intentionally discriminates against minority voters in Hinds County by creating a system of judicial and prosecutorial appointments specifically designed to undermine the historical power of Black residents, through their elected officials, to self-govern.”

Lawyers for the defendants in the case, who include Gov. Tate Reeves and other state leaders, have denied the charges.