David Mixner, a political strategist who helped move gay rights to the center of American politics and put his long friendship with Bill Clinton on the line over the president’s “don’t ask, don’t tell” policy barring gay people from serving openly in the military, died March 11 at his home in Manhattan. He was 77.

He had complications from long-term COVID, according to his friend and executor, Steven Guy.

Mr. Mixner, a farmer’s son from rural New Jersey, lived until he was 30 as a closeted gay person who considered his sexuality a “terrible secret.”

While keeping his personal life intensely private, he built a successful career in Democratic politics. He was working in Los Angeles on the 1977 reelection campaign of Tom Bradley, the city’s first Black mayor, when he decided – with anguishing trepidation – to come out.

Clinton, an old friend who had once let him sleep on his floor in England when the future president was studying as a Rhodes Scholar, was one of the first people Mr. Mixner called on for moral support. They had gotten to know each other in their early 20s at a gathering of opponents to the Vietnam War.

“When I met him when he was young,” Clinton later said, “I thought I had never met a person whose heart burned with the fire of social justice so strongly.”

Mr. Mixner “was an important role model for a lot of gay activists … because he was one of the first who was willing to come out,” said Charles Kaiser, the author of the book “The Gay Metropolis,” a history of gay life in the United States. “It was in a period when whatever your profession was … you mostly assumed that if you came out of the closet it would have a terrible destructive effect on your career.”

A skilled strategist and prolific fundraiser, Mr. Mixner emerged as a leading advocate in the burgeoning movement for gay rights. In 1978, he met with former California governor Ronald Reagan, who would soon run for president, and persuaded him to oppose Proposition 6, a state ballot initiative that would have banned openly gay people from teaching in public schools.

Mr. Mixner appealed to Reagan, a standard-bearer of the conservative cause, on the grounds that the measure would set a dangerous precedent for government intrusion in private life. Reagan later said that while he did not “approve of teaching a so-called gay life style in our schools,” there was “already adequate legal machinery to deal with such problems if and when they arise.” The measure failed.

As the spread of HIV/AIDS became a crisis in the gay community in the 1980s, Mr. Mixner helped spearhead a movement of activists outraged by what they regarded as the scandalously inadequate response by government officials and medical professionals who dismissed the disease as a “gay plague.”

He reached perhaps the peak of his influence with the rise on the national stage of Clinton, who served as Arkansas governor before winning the Democratic nomination for president in 1992. When Clinton decided to run for the White House, it was his turn to call on Mr. Mixner for support.

“Bill, I’ve lost over 180 friends to AIDS,” Mr. Mixner recalled telling him. “Before I can get behind this campaign, I have to know where you stand on this, where you stand on AIDS and our struggle for our freedom.”

Clinton, with guidance from Mr. Mixner, made an unprecedented appeal to gay voters by a major presidential nominee. At a gathering in Los Angeles, he declared to them, “I have a vision of America, and you are part of it.” He pledged to devote federal dollars to HIV/AIDS research, to appoint gay people to prominent government positions and to end the long-standing ban on gays serving openly in the military.

Mr. Mixner was credited with raising $3.4 million for the 1992 Clinton campaign. Clinton kept his pledge in part with his support for HIV/AIDS funding and measures barring anti-gay discrimination in employment, as well as with an executive order that prohibited the federal government from refusing security clearances to gays because of their sexual orientation.

But he left gay rights activists – Mr. Mixner chief among them – bitterly disappointed in 1993 when he announced the “don’t ask, don’t tell” policy, under which military officials were not permitted to ask service members about their sexual orientation, but service personnel had to remain silent about it.

Kaiser, in an interview, observed that the political reality greatly constrained what Clinton could do. At the time, Colin L. Powell, the chairman of the Joint Chiefs of Staff, opposed allowing gays to serve openly in the military, as did many members of Congress.

To defenders of “don’t ask, don’t tell,” the policy was a compromise. But to Mr. Mixner, it was a painful betrayal. Clinton had “sacrificed the freedom of millions for [his] own political expediency,” he said.

Amid the debate over “don’t ask, don’t tell,” Mr. Mixner was arrested at a protest outside the White House and escorted away in handcuffs. He later excoriated Clinton over the president’s decision amid his reelection campaign to sign the 1996 Defense of Marriage Act, which barred federal recognition of same-sex marriage.

“Both Harry Truman and J.F.K. were reluctantly drawn into the civil rights battle, but when they had to choose between political expediency and the moral high ground, both chose the moral high ground,” Mr. Mixner told the New York Times. “Regrettably, this President has not.”

Mr. Mixner said he lost many of his political clients, and that he spent several years in the “wilderness” for the position he took against Clinton. But in light of Clinton’s other efforts on behalf of gay rights, Mr. Mixner reembraced the president before he completed his two terms in office.

“Don’t ask, don’t tell” was repealed under President Barack Obama in 2011, and the U.S. Supreme Court ruled in 2015 that same-sex marriage is protected by the Constitution. Out of office, Clinton renounced his earlier positions on both issues, and Mr. Mixner expressed appreciation for his friend’s evolution.

“The purpose of a movement is to change minds, not in some Stalinistic way to punish those who are not ideologically pure,” he told the Times in 2013. “We created a safe place where he could change his mind.”

Sean Patrick Maloney, who worked as White House staff secretary in the Clinton administration and later became the first openly gay congressman from New York and the chairman of the Democratic Congressional Campaign Committee, said in an interview that “people like me would never have been able to enjoy the careers we have without his sacrifice,” describing Mr. Mixner as a “moral voice who took real risks to win our equality, and very often at a high personal cost.”

“My career, my marriage, so much of what the community now takes for granted,” Maloney added, “we owe to pioneers like David Mixner.”

David Benjamin Mixner, the youngest of three children, was born in Bridgeton, N.J., on Aug. 16, 1946, and grew up in nearby Elmer. His father did agricultural work but never owned a farm. His mother was a homemaker, a notary and a bookkeeper for a John Deere farm-equipment dealership. A brother is Mr. Mixner’s only immediate survivor.

In a memoir, “Stranger Among Friends” (1996), Mr. Mixner described his parents as liberal in many of their beliefs but unaccepting of homosexuality. When a gay schoolmate died by suicide, Mr. Mixner recalled his father remarking that “given what he was, he’s better off dead.”

Mr. Mixner became interested in political activism in high school and dropped out of college to pursue his organizing full-time. He worked for U.S. Sen. Eugene J. McCarthy (D-Minn.), a prominent opponent of the Vietnam War, on his unsuccessful 1968 campaign for president.

In the race for the 2008 Democratic presidential nomination, Mr. Mixner endorsed Obama over Hillary Clinton, then a U.S. senator from New York and the former first lady. He was circumspect in describing the conversation he had with Bill Clinton over the matter, telling the San Francisco Chronicle that “I am very leery of revealing personal conversations with friends because then you’re not a person who can be trusted.”

“But I will say,” he added, “that there was an exchange and an expression of grave disappointment – especially from President Clinton to myself.”

After Mr. Mixner’s death, Bill Clinton offered a tribute to him on X.

“In the more than 50 years since I met David Mixner, I’ve known few people more committed to social justice and so true to their principles,” he wrote. “America is a better, fairer, more inclusive place because of his long fight for LGBTQ+ rights and so much more. His legacy endures in the change he made and the generations of leaders he mentored and inspired.”