Curtis Gans, right, in a 1968 news conference. Another staffer for Sen. Eugene McCarthy’s presidential campaign, Thomas D. Finney Jr., is at left. (AP)

Curtis Gans, a political activist who helped upend the 1968 presidential election with his role in the “Dump Johnson” movement that pushed incumbent Lyndon B. Johnson from the race, and who interpreted many subsequent campaigns as a leading scholar of voter turnout, died March 15 at a hospital in Frederick, Md. He was 77.

The cause was metastatic lung cancer, said his son, Aaron Gans.

Mr. Gans spent nearly his entire adult life in the thick of elections, first as a Democratic organizer and later as a go-to guide for political reporters. For years, journalists relied on the analyses he produced as founder and director of the Washington-based Center for the Study of the American Electorate.

In the early years of his career, Mr. Gans was best known as an associate of Allard K. Lowenstein, the prominent anti-Vietnam War activist who later became a one-term Democratic congressman from New York. He was slain in 1980 by a former protege suffering from mental illness.

Together, Lowenstein and Mr. Gans helped orchestrate a historic event in U.S. politics: the withdrawal of a sitting president from a reelection race.

Johnson, who assumed office after the assassination of John F. Kennedy in 1963, won a landslide victory in 1964 against Republican Barry Goldwater and appeared well positioned for the 1968 campaign. But as the United States escalated its involvement in Vietnam, the president became increasingly identified with an unpopular war. To Lowenstein and Mr. Gans, he seemed vulnerable to a contest from within his party.

By 1967, they had launched an effort to recruit another Democrat to challenge Johnson on an antiwar platform. Several politicians initially declined, including Robert F. Kennedy, the New York senator and brother of the late president, and George S. McGovern, the South Dakota senator.

Sen. Eugene J. McCarthy of Minnesota ultimately agreed to run. In March 1968, with Mr. Gans as one of his chief campaign staffers, he won a shocking 42 percent of the vote in the New Hampshire primary. (Johnson took 49 percent.)

Days later, sensing Johnson’s weakness, Kennedy declared that he also would seek the nomination. With that development, Johnson announced that he would not seek and would not accept the nomination for another term in the White House.

In the following months, Kennedy was assassinated in Los Angeles and McCarthy lost the Democratic nomination to Vice President Hubert Humphrey at the tumultuous Chicago convention. Humphrey went on to lose the general election to Republican Richard M. Nixon.

Mr. Gans became a political commentator and, in 1976, helped start the Center for the Study of the American Electorate. Now associated with American University, it began as a no-frills operation functioning at one point with an electric typewriter and a broken photocopier.

He was called a “Chicken Little” of the voter turnout forecasting industry. As he tracked the waxing and waning of poll numbers, he drew attention to what he saw as the increasing disengagement of American voters.

“A Cassandra among the revellers,” the Toronto Globe and Mail wrote in a 2000 profile, Mr. Gans “sees a nation choking on its prosperity, intoxicated by its sense of social well-being . . . but tragically blind to the erosion of its civic institutions.”

Unlike some other political observers, he did not cite voter registration or other mechanics as the key drivers of voter turnout.

“The sad fact of our political life,” he told The Washington Post in 1994, “is that the combination of perceived ineffective government and corrosive and vacuous elections is destroying both the citizenry’s will to participate and its faith in the utility of our political institutions and the political process itself.”

Curtis Bernard Gans, a son of German-Jewish immigrants, was born June 17, 1937, in Manhattan. His father was a businessman and his mother was a masseuse.

Mr. Gans was a 1959 graduate of the University of North Carolina at Chapel Hill, where he was editor of the student newspaper. He became involved in the civil rights movement and was an early member of Students for a Democratic Society. In the 1950s and 1960s, he and Lowenstein held leadership roles with the National Student Association.

For several years Mr. Gans worked for the United Press International wire service and reported from Dallas in the aftermath of President Kennedy’s assassination. His commentaries appeared over the years in publications such as The Post, the New York Times and USA Today, and his books included the 900-page “Voter Turnout in the United States, 1788-2009.”

Mr. Gans was a Lovettsville, Va., resident. His marriages to Eugenia Grohman and Shelley Fidler ended in divorce. Besides his son, who lives in the District, survivors include a brother.