Richard S. Williamson, a Chicago lawyer who served as an aide to Republican presidents and lost a U.S. Senate race to Carol Moseley Braun, died Dec. 8 at a hospital in Evanston, Ill. He was 64.

The cause was complications from a cerebral hemorrhage, said Alan Holmer, a family friend.

One of Mr. Williamson’s best-known undertakings was a nationally watched Senate race against Moseley Braun, a Chicago Democrat, in 1992. By winning, she became the first black woman to serve in the Senate.

In the early 1980s, Mr. Williamson had served President Ronald Reagan as White House staff assistant to the president for intergovernmental affairs. President George W. Bush appointed Mr. Williamson U.S. ambassador to the United Nations Commission on Human Rights and later special envoy to Sudan, where he worked to prevent genocide.

In July, he authored a report on genocide prevention with former secretary of state Madeleine Albright. “As we know from our experience in government service, once the body counts begin to mount, our options for responding immediately become far less palatable politically,” they wrote.

Mr. Williamson was a senior policy adviser for the presidential campaigns of Republicans John McCain and Mitt Romney.

Richard Salisbury Williamson was born on May 9, 1949, in Evanston, Ill. He was a 1971 graduate of Princeton University and a 1974 graduate of the University of Virginia School of Law.

He was a resident of Kenilworth, Ill., and a longtime leader in the Illinois GOP. He chaired the state Republican Party from 1999 to 2001 and since 2010 had been a member of the Republican National Committee.

Survivors include his wife, the former Jane Thatcher, and three children.