WASHINGTON (AP) _ Jeane J. Kirkpatrick is an outspoken former ''lifelong Democrat'' who completed her transformation into a Republican less than three years ago.

She registered as a member of the GOP in early 1985 after ending four years as President Reagan's ambassador to the United Nations, where she had a reputation as a hard-liner on foreign policy and was a lightning rod for the opposition.

On Sunday, Mrs. Kirkpatrick announced she would not make a bid for the 1988 Republican presidential nomination, but would be willing to fill the vice presidential slot on a GOP ticket.

Mrs. Kirkpatrick, 60, won a starring role at the Republican National Convention in 1984 at a time when she was still a registered Democrat.

She didn't disappoint her new friends and used her convention speech to denounce a dismal period of retreat and decline'' during the Carter administration. She labeled Democrats the ''blame-America-first crowd.''

Since returning to academic life, she has remained a darling of conservative Republicans.

Mrs. Kirkpatrick was born Jeane Duane Jordan on Nov. 19, 1926, the daughter of Oklahoma oil wildcatter Welcher F. Jordan. Her mother, the former Leona Kile, was his accountant.

She graduated from Barnard College, the women's division of Columbia University, and later earned master's and doctorate degrees in political science from the university. In 1955 she married Evron M. Kirkpatrick, 15 years her senior and also a political scientist. She had worked for him while serving as a research analyst at the State Department in the early 1950s. They have three sons.

Her husband had managed Hubert Humphrey's 1945 campaign for mayor of Minneapolis, and Mrs. Kirkpatrick later described herself as a Democrat in the mold of Humprey, the late Democratic vice president, and the late Sen. Henry M. ''Scoop'' Jackson, a conservative on defense and national security issues.

She told the 1984 Republican convention that she was a ''lifelong Democrat,'' and she previously described herself as a ''welfare-state liberal'' on domestic policy.

She was a professor at Georgetown University when she came to the attention of presidential candidate Reagan. Her article attacking Carter's foreign policy appeared in the neoconservative magazine Commentary, and it was shown to Reagan by Richard V. Allen, who later became Reagan's first national security adviser.

In it she accused the Carter administration of a double standard that favored leftist revolutionary movements over rightwing dictatorships, resulting in the undermining of friendly authoritarian regimes by more repressive leftists. Nicaragua and Iran were examples.

''This is incredible. Who is this person?'' was Reagan's favorable reaction, as recounted by Allen.

She was enlisted to the campaign's foreign policy team and, after the election, her nomination to the U.N. job was unanimously confirmed by the Senate.

Some U.N. diplomats were offended by her bluntness and what they regarded as a lecturing, professorial tone. She wasn't particularly taken with her new U.S. colleagues either. She complained of ''rank sexism'' in the male- dominated body.

But she seemed to enjoy the international forum and the controversy she kicked up.

She once remarked that American missionaries slain in El Salvador were ''not just nuns but political activists.'' And the Congressional Black Caucus demanded her dismissal in 1981 for meeting with the chief of military intelligence of South Africa's apartheid government.

She was met by college protesters in an era marked by campus calm. She turned down honorary degrees in the face of objections from students and professors.

Her tenure was marked by friction within the administration. She clashed with Reagan's first secretary of state, Alexander Haig, now a GOP presidential hopeful. She also differed with his successor, George Shultz.

''I'm sure that Alexander Haig thought he was going to wipe me out during the first nine months, and he didn't,'' she once said.

She reportedly was angered when passed over for other administration posts, and she angled for a better job after Reagan's 1984 re-election. Reagan was quoted as saying he had no job available that was ''worthy of her.''

When she left the U.N. job, she said it was to return to academic life. She called her term ''a consciousness-raising experience'' that made her aware of the ''double bind'' women encounter in the foreign policy arena.''