William E. Dannemeyer, an immensely popular former California congressman whose long and implacable opposition to liberal causes cemented Orange County’s reputation as a citadel of conservatism in the 1980s, died on July 9 in Thousand Palms, Calif. He was 89.

The cause was complications of dementia, his son, Bruce, said.

In his seven terms in the House of Representatives, Mr. Dannemeyer vigorously opposed, among other things, higher taxes, environmentalism, legal abortion, and civil rights protections for gay people and AIDS patients.

“He helped shape modern conservatism,” Fred M. Whitaker, the Republican county chairman, said in a statement after Mr. Dannemeyer’s death. “His leadership in grass-roots organizing earned Republicans every partisan seat in the county by the time he retired at the end of 1992.”

Mr. Dannemeyer ran in every congressional election from 1978 to 1990 and never got less than about 70 percent of the vote in his district, in the southeastern suburbs of Los Angeles.