Rosemary Stambaugh, widow of Del. Warren G. Stambaugh, announced yesterday that she will seek the Democratic nomination for his seat in the Virginia House of Delegates, setting up what is expected to be a heated race with civic activist Karen Darner. Darner announced her candidacy for the nomination over the weekend.

"Warren left an unfinished agenda I think needs to be completed," Stambaugh said yesterday, as she cited her husband's work on mental health, education and transportation and on behalf of the disabled.

Other potential Democratic candidates have bowed out, leaving Darner and Stambaugh to face each other in a Democratic Party caucus Dec. 5. The general election for the South Arlington seat is Jan. 8.

Darner, 45, a speech and hearing therapist for the Arlington schools, is a longtime member of Arlington's political establishment and a former Richmond legislative aide. She recently returned from two years in the Peace Corps.

Stambaugh, 44, is a former congressional staff director, federal employee and local political activist. For the last two years she has owned a business, Got It Maid Inc., a residential cleaning service.

She acknowledged she cannot match Darner's civic re'sume', saying her energies for many years were devoted to her job and two children, who are now grown. But, she said, "I'm a known quantity in Richmond."

As a senior staff assistant of the House Post Office and Civil Service Committee in 1979, Stambaugh also said she led an investigation into sexual harassment in the federal government, which resulted in federal guidelines on the problem.

Darner is a past president of the Mental Health Association of Northern Virginia and of the Arlington League of Women Voters. She is a former aide to state Sen. Edward M. Holland (D-Arlington).

"I've been involved in the community for a very long time, trying to make life better for Arlington residents," she said.

She said her experience in Richmond and as a lobbyist on behalf of causes such as mental health make her "highly qualified" for the post.

Candidates for the Republican nomination, Alice Tennies, a schools activist and critic of sex education, and J.D. Millar, the vice president of an international trading firm, will face each other in a party canvass Saturday.