Harry V. Jaffa, who explored America’s founding in many books, but shifted modern politics with two speech lines that cast Senator Barry M. Goldwater as an extremist, abetting his landslide defeat in the 1964 presidential race and the birth of a zealous new conservatism, died on Saturday. He was 96.

The Claremont Institute in California, where he was a distinguished fellow, announced his death on its website, giving no other details.

A professor and author of political histories, Dr. Jaffa traced the nation’s origins to the philosophies of Aristotle and John Locke and analyzed the Declaration of Independence, the Constitution, and the contributions of Washington, Jefferson, Madison and other founders. He was also a Lincoln scholar, and his book on the 1858 Lincoln-Douglas debates is regarded as a classic in its field.

A disciple of the political philosopher Leo Strauss, Dr. Jaffa devoted much of his academic career to interpretations of equality, liberty, ethics, morality and patriotism, and he clashed with fellow conservatives over their meaning and the framers’ intentions. He took conservative rivals to task for ignoring “natural law,” which he called a concept of justice common to all mankind and a key to America’s foundation.

Though not widely known outside academic circles, Dr. Jaffa wrote for National Review and other publications and was an astute analyst of conservative politics. In 1964, the Goldwater campaign invited him to join the candidate’s brain trust. He was thus in the Cow Palace outside San Francisco when the Republican National Convention nominated the Arizona senator to run for president....