Irving Kristol, the political commentator who, as much as anyone, defined modern conservatism and helped revitalize the Republican Party in the late 1960s and early ’70s, setting the stage for the Reagan presidency and years of conservative dominance, died Friday in Arlington, Va. He was 89 and lived in Washington.

His son, William Kristol, the commentator and editor of the conservative magazine The Weekly Standard, said the cause of death was complications of lung cancer.

Mr. Kristol exerted an influence across generations, from William F. Buckley to the columnist David Brooks, through a variety of positions he held over a long career: executive vice president of Basic Books, contributor to The Wall Street Journal, professor of social thought at New York University, senior fellow at the American Enterprise Institute.