U.N. Ambassador Jeane J. Kirkpatrick, a lifelong Democrat whose tough foreign policy stance has made her the darling of conservative Republicans, plans to switch parties early next month, and GOP officials have scheduled a coming-out party in her honor.

Kirkpatrick, who will leave the Reagan administration at the end of this month to return to teaching and writing, is to make her debut as a Republican at a fund-raiser for the new GOP Women's Political Action League (GOPAL) on April 3.

"I'm not denying it," Kirkpatrick said yesterday, according to the Associated Press. "I've been making fairly clear that I had to think seriously about bringing my formal registration into line with my behavior and my views."

The invitations to the $150-a-head fund-raiser show a dancing elephant on the cover and read, "Welcome to the Party Jeane Kirkpatrick!"

GOP officials said Maureen Reagan, the president's elder daughter and a founder of GOPAL, was instrumental in arranging the setting of Kirkpatrick's party conversion. She and Kirkpatrick reportedly have become friends during the Reagan presidency.

"The day of the event, she Kirkpatrick will re-register as a Republican," said Suzanne Rich, administrative assistant to Maureen Reagan, who is in Europe on business. "That's why our event is that day."

Sponsors of the event said that they expect more than 500 people, including several Cabinet members, to attend and that interest is so great that they have had to move the event to a larger room. The fund-raiser will be followed by a dinner honoring Kirkpatrick.

The Republicans have been unabashedly wooing the popular Kirkpatrick. Her party switch has been the subject of discussion since her speech to the Republican National Convention in Dallas last August.

Kirkpatrick, whose Democratic mentors included the late senators Hubert H. Humphrey (Minn.) and Henry M. Jackson (Wash.), brought roars from the Republicans in Dallas with a speech in which she blistered the Democrats as the "blame America first" party.

She has since become one of the stars on the Republican Party circuit. At a political pep rally for Reagan appointees earlier this year that included President Reagan and members of his Cabinet, Kirkpatrick drew the loudest applause -- even though she did not speak.

Last month, she was featured at a GOP Lincoln Day dinner in New York City, where her anticipated conversion to the GOP was openly discussed. At the time, Kirkpatrick deflected the talk by saying that she had attended only to have dinner "with so many of my friends."

GOP officials said Kirkpatrick did not want to switch parties until she had left her U.N. post.

Kirkpatrick is returning to the academic world at the end of the month because she was unable to land a top foreign policy-making post in the administration. With the backing of leading conservatives in the party, she spent several months trying to find such a position but reportedly was opposed by Secretary of State George P. Shultz and Treasury Secretary James A. Baker III, then White House chief of staff.

Conservative Republicans already are touting her as a potential vice presidential candidate in 1988.

GOPAL is a political action committee that will contribute money to female Republican candidates. In addition to Maureen Reagan, its founders include Washington lobbyist Nancy Clark Reynolds and former Housing and Urban Development secretary Carla Hills.