Ford's Theatre raised the curtain yesterday on what may be its most important attraction of 1986: a larger and more substantial oak chair.

The new version of a 19th-century side chair will replace three earlier cane-bottom designs (copied from mid-19th-century Mathew Brady photographs), which date from the Ford's restoration in 1968.

House Speaker Thomas P. (Tip) O'Neill Jr., an accomplished chairman, tested and approved the new design and pronounced it more fitting to a man of his stature. During 18 years of spectating in the earlier cane-bottom models, he said, he worried about imminent collapse.

"Ford's is (his wife) Millie's favorite charity," he said, "and we've been here -- I guess 50 times -- with presidents and other dignitaries. There's always a point of discussion afterward -- how good the show was, how uncomfortable the chairs."

Frankie Hewitt, Ford's executive producer, admitted that hatred of the chairs is "one of the few issues in Washington everyone agrees on."

Hewitt is hoping that friends of Ford's care enough to give $500 donations to help pay for the new chairs -- for which, in return, they'll get one of the old, uncomfortable chairs, complete with a brass plaque certifying it from Ford's. Hewitt said former senator Howard Baker has already made his donation and taken his souvenir chair home: "He says he put it by the telephone, to cut down on his habit of talking too long . . . Talk show host Merv Griffin bought 10 chairs for his conference room to shorten meetings." O'Neill said he and his wife will donate one to the North Cambridge Catholic High School in Massachusetts, where they met.

Senator Alan K. Simpson (R-Wyo.), assistant majority leader, recalled sitting on the old chairs and finding them "most painful." While he and O'Neill appear physically much the same, "our upholstery is different," Simpson noted.

Humorist Mark Russell, who has played Ford's several times, sang an ode to the new chair: "With these new chairs, Ford's really cares -- and that's the bottom line."

The 800 new chairs, wider by as much as four inches and deep, with taller backs than their predecessors, have padded seats and a back pillow, both in a beige velvet. Priced at $225 each, they're scheduled to be screwed to the floor of the theater in time for the opening of "Hot Mikado" on March 18.

Joseph Blanchfield, owner of the 11-person firm that manufactures the new chair, said he's thinking about marketing copies of the new chair to the public, for $400 to $500 each, with a plaque saying "Ford's Theatre Classic."