WASHINGTON (AP) _ The 70th birthday party for Katharine Graham, chairman of The Washington Post Company, attracted 600 guests from across the political and media spectrums, including one of the newspaper's frequent subjects - President Reagan.

''Here's looking at you, kid,'' Reagan toasted Mrs. Graham in his best Humphrey Bogart imitation at Tuesday night's black-tie birthday bash.

The president called Mrs. Graham a ''sensitive, thoughtful and very kindly person.''

Among the invited guests from the Reagan administration were Secretary of State George P. Shultz, Secretary of Defense Caspar Weinberger and Treasury Secretary James A. Baker III, as well as the president and his wife, Nancy.

Representing Congress were Speaker of the House Jim Wright, D-Texas; senators Alan Simpson, R-Wyo.; William Cohen, R-Maine; Edward Kennedy, D- Mass.; Bill Bradley, D-N.J.; Richard Lugar, R-Ind., and John Heinz, R-Pa.

The guests feasted on salmon, beef filet with cucumber sauce, summer vegetables, birthday cake, sherbet and berries.

Publisher Malcolm Forbes pulled out a bottle of Chateau Lafite Rothschild from a tote bag bearing the title of his magazine, Forbes. He said the bottle was laid down the year Graham was born.

Also in attendance at the party were Rupert Murdoch, chairman of the News Corp. Ltd.; Frank Batten, chairman and chief executive officer of Landmark Communications Inc. and former chairman of The Associated Press; Otis Chandler, chairman of the executive committee of the board of directors of Times Mirror; William Paley, chairman of CBS; Charles Peters, editor in chief of Washington Monthly and Arthur O. Sulzberger, publisher of The New York Times.

Mike Wallace of CBS said Graham was ''a woman who in effect, I suppose, came to the job unprepared and turned out to be one of the giants of journalism in the last quarter century.''