Morton Janklow, one of the nation’s most powerful literary agents who elevated the power of the profession in advocating for authors, died Wednesday morning of heart failure at his home in Water Mill, N.Y.. He was 91 years old.

Janklow’s clients included the likes of Sidney Poitier, Sidney Sheldon, Danielle Steel, Jackie Collins, Nancy Reagan and Ted Turner. His death was confirmed by publicist Paul Bogards to The New York Times.

Janklow began his career as a literary agent in 1972 when his client and friend William Safire asked him to help with a book he was writing about President Richard Nixon. The corporate attorney educated himself on the publishing industry and successfully negotiated a contract for Safire’s book. After the Watergate scandal broke, the book’s publisher attempted to back out of the $250,000 contract.

Janklow sued the publisher for arbitration, ushering in a new age for the publishing industry. He recaptured the rights to the book and struck a deal with another publisher. Notably, Janklow helped break publishing precedent by recovering about one-third of Safire’s advance. Before this, authors usually had to return all the money once a publisher dropped their book. Safire’s book, “Before the Fall: An Inside View of the Pre-Watergate White House” became a bestseller.

From then on, Janklow became known for advocating for authors. By the1980s, he was routinely securing multimillion-dollar contracts for writers, including several deals that exceeded $25 million. Due in large part to Janklow, agents became the first to see the work of unknown authors. It was an agent’s judgments, often based on sales potential and not public interest, that largely determined what publishers bought and presented to the public.

“We took the publisher out of the captain’s seat and put the author in it,” he writes on his firm’s website. “The publisher is replaceable; the author is not.”

In 1977, Janklow established his own literary agency where he would later work with numerous acclaimed authors, including winners of the National Book Award and Pulitzer Prize. His clients included Pope John Paul II, Barbara Walters, John Glenn, Al Gore, Thomas Harris, Judith Krantz, John Erhlichman, David McCullough, Ronald Reagan, Michael Moore, Pat Riley and Carl Sagan.

He co-founded the New York-based Janklow & Nesbit Associates with Lynn Nesbit in 1989, where he served as chairman until his death.

Janklow was born in New York City in 1930 and raised in Queens. The son of a lawyer, Janklow graduated from Syracuse University in 1950 and Columbia Law School in 1953. At Syracuse University, he founded the Janklow Arts Leadership Program and served on the College of Arts and Sciences Board of Visitors. He also founded the Morton L. Janklow Program for Advocacy in the Arts and endowed the Morton L. Janklow Chair in Literary and Artistic Property Law at Columbia University. Janklow also taught in the program.