He had dementia, said his son, Bruce Dannemeyer.
Mr. Dannemeyer was “a symbol of the all-white, straight, Christian Orange County,” which became a nationally recognized bastion of conservatism in the late 1960s, said Fred Smoller, a political science professor at Chapman University in Orange, Calif.
A lawyer and former conservative Democrat, Mr. Dannemeyer served in the U.S. House as a Republican from 1979 to 1993, representing Fullerton and other southeastern suburbs of Los Angeles when the region was dominated by defense contractors and former Midwesterners.
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He was part of an outspoken line of far-right politicians to emerge from the county, including congressmen James B. Utt, who warned that “barefooted Africans” were training in Georgia to help the United Nations take over the United States; John G. Schmitz, a leader of the John Birch Society; Robert K. Dornan, an antiabortion firebrand known as “B-1 Bob”; and Dana Rohrabacher, a protege of President Ronald Reagan who later backed Russian President Vladimir Putin.
A Lutheran elder who said he saw little separation between church and state, Mr. Dannemeyer was often cited as one of the most conservative members of Congress and routinely took to the House floor to lambaste the “homosexual movement,” abortion rights activists, racy television programs and “environmental zealots.”
He favored regulatory rollbacks, expansive offshore oil drilling, cuts to social programs and a return to the gold standard. And he embraced controversy by promoting false claims about HIV/AIDS, reading a graphic description of “the average homosexual’s favorite activities” into the congressional record, and likening Nelson Mandela to black militant H. Rap Brown and felon Willie Horton.
Mr. Dannemeyer regularly broke with his GOP colleagues, including when he voted against the Americans With Disabilities Act, which barred discrimination against AIDS patients, and opposed legislation tracking hate crimes because it placed homosexuality on par with race, religion and ethnicity as protected classes.
His fire-and-brimstone rhetoric on gays made him a hero to the religious right and a leading villain to LGBTQ rights advocates, with whom he delighted in publicly debating, often declaring, “God’s plan for man in this world is Adam and Eve, not Adam and Steve.”
Mr. Dannemeyer advocated for new anti-sodomy laws, falsely declared that AIDS patients “emit spores” that can infect pregnant women and followed conspiracy theorist Lyndon H. LaRouche Jr. in calling for AIDS patients to be quarantined. Twice, he unsuccessfully sponsored California ballot propositions requiring health officials to compile the names of people with HIV and remove them from some jobs.
In a 1989 book, “Shadow in the Land,” he warned: “We must either defeat militant homosexuality or it will defeat us.”
Mr. Dannemeyer received widespread support from his district, easily winning reelection every two years, but proved too radical to win statewide office. In 1992, he decided to compete for a Senate seat after the incumbent, Pete Wilson (R), was elected governor of California. He lost in the primary to John Seymour (who fell in turn to Dianne Feinstein, the Democrat) and unsuccessfully ran again in 1994.
Amid an influx of nonwhite residents and college graduates, Orange County swung toward the Democratic Party, voting for presidential nominee Hillary Clinton in 2016. Two years later, Democrats flipped four of its congressional districts to win all six districts in the county.
“It’s the passing of an era,” said Smoller, the political scientist. “We are 2,400 people away from Democrats passing Republicans in voter registration. By October, Orange County will be blue.”
The only son of German immigrants, William Edwin Dannemeyer was born in Long Beach, Calif., on Sept. 22, 1929. His father, who was epileptic, ran a feed store in Los Angeles before moving to a sanitarium in Nebraska when William was a young boy, leaving his mother to care for him and an older sister by scrubbing floors.
“The closest thing he had to a mentor — or a father, for that matter — was his scoutmaster,” said Bruce Dannemeyer. “Boy Scouts filled a large void in his life.”
Mr. Dannemeyer, an Eagle Scout, received a bachelor’s degree in 1950 from Valparaiso University, a Lutheran school in Indiana. He later said he developed the “bug” to run for office while on an internship in Washington and graduated from Hastings College of the Law, a branch of the University of California in San Francisco, in 1952.
After serving in Europe with the Army Counter Intelligence Corps, Mr. Dannemeyer was a deputy district attorney in Santa Barbara County. In the late 1950s, he moved to Fullerton, starting his own law practice and serving as assistant city attorney.
Mr. Dannemeyer was elected to the California State Assembly as a Democrat in 1962 and 1964, then switched parties after losing a State Senate race in 1966. He served as a municipal and superior court judge pro tempore, was reelected to the State Assembly in 1976 and two years later successfully ran for Congress.
In a statement, the Republican Party of Orange County credited Mr. Dannemeyer with leading a grass-roots effort to win “every partisan seat in the county” by the time he left office. He later lobbied for a constitutional amendment protecting school prayer, and he espoused a conspiracy theory linking President Bill Clinton to two-dozen “frightening” deaths, calling for a congressional hearing into the possibility that Clinton may have had his political opponents murdered.
Mr. Dannemeyer drifted further to the fringes after his wife of 44 years, the former Evelyn Hoemann, died in 1999, his son said. In 2004, he married Lorraine Day, a Holocaust denier and advocate of alternative medicine; he later promoted anti-Semitic conspiracy theories and other views that Bruce Dannemeyer described as “unpleasant and plain wrong.”
In addition to his wife, survivors include three children from his first marriage, Bruce Dannemeyer of Rancho Santa Margarita, Calif., Kim Davis of Anaheim, Calif., and Susan Hirzel of Fullerton; 10 grandchildren; and four great-grandchildren.
“Our state and our institutions are emphatically Christian,” Mr. Dannemeyer once said, defending his views on church and state. “This is a religious people. You can’t get any clearer than that.”
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