While attempting to register to vote in Louisiana in the early 1950s, Lucius J. Barker was required to take a test about his understanding of the Constitution. The test, rarely administered to white residents, was designed to discourage black citizens from voting.

Dr. Barker, then a graduate student of political science, answered most of the questions with ease. When the registrar asked him to explain the due process clause of the 14th Amendment, which provides that no one should be deprived of “life, liberty, or property, without due process of law,” Dr. Barker said he couldn’t give an answer.

“You don’t know?” the registrar asked.

“No, I don’t know, and neither does the Supreme Court,” Dr. Barker replied.

He then cited several cases in which the court interpreted the clause in different ways — and deliberately mentioned unrelated cases to confirm his suspicion that the white registrar did not understand the questions he was asking.

Dr. Barker was ultimately successful in his effort to register, but for years afterward, he had his students at Washington University and Stanford University take the Louisiana understanding test, as it was called, to demonstrate the obstacles faced by African Americans seeking to vote.

After attending segregated schools in his home state of Louisiana, Dr. Barker became a leading scholar of the political implications of race, civil liberties and the judicial system. He wrote two influential political science textbooks, and his students at Stanford included Sen. Cory Booker (D-N.J.) and twin brothers Julián Castro, a former U.S. housing and urban development secretary, and Rep. Joaquin Castro (D-Tex.).

Dr. Barker died June 21 at his home in Menlo Park, Calif., at age 92. The cause was complications from Alzheimer’s disease, said his daughter Heidi Barker.

Throughout his career, Dr. Barker examined the role of the courts in the country’s political system, as well as the political effects of race. He was known for developing the idea of the Supreme Court as a “safety valve,” resolving sensitive issues that the legislative branch could not.

In 1970, he and his older brother, political scientist Twiley W. Barker Jr., published the first edition of “Civil Liberties and the Constitution,” which provided historical context and what Dr. Barker called “systemic perspective” for Supreme Court rulings on civil liberties cases.

“We tried to make it come alive, to let people know that court cases don’t just spring out of nowhere,” Dr. Barker told the Chicago Tribune in 2009.

The book, now in its ninth edition, is still used in college courses and is “a classic, a seminal work,” Duke University professor Paula D. McClain said in an interview.

In 1976, Dr. Barker published “Black Americans and the Political System,” which offered a comprehensive view of black political experience through history. Revised several times and later titled “African Americans and the Political System,” it became a standard text.

After years of studying politics from afar, Dr. Barker took a more active role in 1984, when he supported the Rev. Jesse Jackson’s failed presidential bid and served as a delegate to the Democratic National Convention. In a 1988 book, “Our Time Has Come,” Dr. Barker wrote of how he was “converted from a cloistered scholar to an open activist delegate willing to go all the way for the Jackson cause.”

He later participated in Jackson’s 1988 campaign and in Barack Obama’s successful run for the presidency in 2008.

Beyond his writings, Dr. Barker’s influence was deeply felt in the classroom and in academic circles.

“He was a model and an inspiration,” Booker, who has two degrees from Stanford, said in a statement. “He taught me the importance of rigorously pursuing knowledge, and using that knowledge in the service of others. And he lived this ethos, with a generosity of heart that nurtured, encouraged, and guided me toward a career of public service.”

Lucius Jefferson Barker was born June 11, 1928, in Franklinton, La. Both of his parents were teachers, and his father became a principal in the then-segregated schools of Louisiana.

At Southern University and A&M College, a historically black institution in Baton Rouge, Dr. Barker switched his academic interest from medicine to political science, graduating in 1949. He received master’s and doctoral degrees in political science in 1950 and 1954, respectively, from the University of Illinois at Urbana-Champaign.

He taught at Southern University, the University of Wisconsin at Milwaukee and the University of Illinois for two years before joining the faculty at Washington University in St. Louis in 1969. He moved to Stanford in 1990 and was a visiting professor at other universities, including Harvard, before retiring from teaching in his late 70s.

His wife of 55 years, the former Maude Beavers, died May 19. Survivors include two daughters, Tracey Barker-Stevens of Los Angeles and Heidi Barker of Chicago and Miami; and two grandsons. His brother, a longtime professor at the University of Illinois at Chicago, died in 2009.

Dr. Barker was elected to the American Academy of Arts and Sciences in 1994 and chaired the political science departments at both Washington and Stanford universities. In 1992, he became the second African American president of the American Political Science Association. (The first was Ralph Bunche, who received the 1950 Nobel Peace Prize.)

“Lucius had this outsized personality,” McClain, the current president of the American Political Science Association, said. “He found a commonality with people wherever he could. He made sure he was opening doors for as many people as he could.”