Robert A. Dahl, an influential political scientist who championed democracy in theory and critiqued it in practice in such books as “Who Governs?,” died Feb. 5 in Hamden, Conn. He was 98.

His family announced the death but did not disclose the cause.

A professor emeritus at Yale University, Dr. Dahl was best known for the 1961 publication “Who Governs?” It was cited by the Times Literary Supplement as among the 100 most influential books since World War II.

“Who Governs?” probed the political system of Dr. Dahl’s own community at the time, New Haven, which he considered an ideal microcosm of the country: two strong parties, a long history and a careful progression from patrician rule to self-made men to party rule, where candidates of varied ethnic and economic backgrounds — a garage owner, an undertaker, a director of publicity — might succeed.

Dr. Dahl wanted to know who really ran the city, and, by extension, the country. Sociologist C. Wright Mills, in “The Power Elite,” had written that wealth and power were concentrated within a tiny group of people.

Robert A. Dahl in 2005 (Yale University )

Dr. Dahl believed that no single entity was in charge. Instead, there were competing ones — social, economic and political leaders whose goals often did not overlap. He acknowledged that many citizens did not participate in local issues and that the rich had advantages over the poor, but he concluded that New Haven, while a “republic of unequal citizens,” was still a republic.

Dr. Dahl’s conclusions were strongly challenged in the 1970s by sociologist G. William Domhoff, who used research provided in part by Dr. Dahl himself to find that he had underestimated the power of the business community and overestimated the divisions among New Haven’s leaders. Domhoff asserted that Dr. Dahl relied too much on the people with whom he spoke.

“It may be that the most serious criticism I can make of Dahl is that he never should have done this interview-based study in the first place, for it was doomed from the start to fall victim to the ambitions and plans of the politicians, planners, lawyers, and businessmen that he was interviewing,” Domhoff wrote.

Dr. Dahl himself tried to bring more democracy to the Yale campus, especially during the uprisings of the 1960s and ’70s. In 1965, he headed a committee that gave students a role in granting tenure to faculty members. Six years later, he chaired a committee that led the school to start a program for African American studies.

Robert Alan Dahl was born in Inwood, Iowa, on Dec. 17, 1915. His father, a doctor, moved the family to the territory of Alaska during the Depression. He graduated from the University of Washington in 1936 and received a doctorate in political science from Yale in 1940.

During World War II, he was an economist on the War Production Board and then served in the Army in Europe. His decorations included the Bronze Star Medal. He joined the Yale faculty in 1946.

He became president of the American Political Science Association and won several awards, including the Talcott Parsons Prize for social science. His other works included a popular textbook, “Democracy in the United States,” and “Polyarchy,” a term coined by Dr. Dahl for a modern, decentralized democracy.

Dr. Dahl established several conditions he considered necessary for democracy (and polyarchy), including freedom from foreign interference, the peaceful transfer of power and the right to vote.

In recent years, he increasingly questioned whether any existing government fully deserved to be called democratic. He worried that economic inequality made political equality impossible yet believed that open markets were needed to counteract government control.

In the 2001 book “How Democratic is the American Constitution?,” he criticized the framers as elitist and shortsighted for not ensuring women’s rights and for counting slaves as three-fifths of a person when determining a state’s right to representation.

“This was simply diabolical, because to the insult of defining a person held in bondage as three-fifths of a human being it added the injury of using that definition to augment the political power of that person’s oppressors,” Dr. Dahl wrote. “Wise as the framers were, they were necessarily limited by their profound ignorance.”

In 1989’s “Democracy and Its Critics,” Dr. Dahl examined the evolution of democracy from ancient Greece to the present. He noted how fully participatory democracy was only possible in a small community such as Athens (where women and slaves could not vote) and documented how political thinkers came to accept that representation by elections was the only way to sustain democracy in a larger country.

Using Socratic dialogues, he also considered the arguments of those who opposed democracy, from anarchists arguing for no government to “guardians” supporting rule by a well-trained elite. Dr. Dahl explored what made a system democratic, what endangered a democracy and what ensured its survival. He welcomed the rise of so many democracies in the latter half of the 20th century, from Eastern Europe to South America, but doubted whether any country could meet all of democracy’s goals.

“Yet the vision of people governing themselves as political equals,” he wrote, will remain “a compelling if always demanding guide in the search for a society in which people may live together in peace, respect each other’s intrinsic equality, and jointly seek the best possible life.”

His first wife, the former Mary Bartlett, died in 1970. A son from that marriage, Peter Dahl, died in 1977.

Survivors include his wife of 40 years, Ann Sale Barber of Hamden; three children from his first marriage, E. Kirsten Dahl of New Haven, Eric Dahl of Santa Barbara, Calif., and Christopher Dahl of Portland, Ore.; two stepdaughters, Sara Connor of Sturbridge, Mass., and Jane Thery of Washington; and four grandchildren.

Adam Bernstein contributed to this report.