Catherine (Cassie) Mackin, 43, a Washington correspondent for ABC News who was noted for her knowledge of politics and government, died of cancer Nov. 20 at the home of a relative in Towson, Md.

Miss Mackin joined ABC in 1977. She had worked for the Hearst newspaper chain in Washington from 1963 to 1969. She began her television career at WRC-TV in Washington, where she worked from 1969 to 1971. She then became a correspondent for NBC News.

For all of these organizations she covered national politics and government news in Washington.

At ABC, she was a Capitol Hill correspondent. In 1980, she covered Sen. Edward M. Kennedy's unsuccessful bid for the Democratic presidential nomination. She later helped cover the Democratic National Convention and reported on major Senate races. More recently, she had contributed to "20/20," ABC's weekly news magazine, and had taken part in David Brinkley's "Roundtable Discussion" shows.

Roone Arledge, the president of ABC News and Sports, said: "Cassie's courage over this past year and a half of illness was an inspiration to us all. She was a wonderful woman and a superb journalist."

Miss Mackin was born in Baltimore. She was an honors graduate of the University of Maryland and a Nieman Fellow at Harvard University in 1967 and 1968.

Survivors include her mother, Catherine Mackin of Baltimore; two sisters, Margaret Mackin Ruth of Towson, and Michaeline Mackin O'Neill of Baltimore, and two brothers, Michael J. Mackin of Baltimore, and Jerard J. Mackin of New Orleans.