Candidates for the Virginia House of Delegates are practically stumbling over each other in Fairfax County, where 29 Democrats and Republicans are competing in three crowded primaries and a fourth contest that was ordered at the last minute.

Eight of the candidates are incumbents, and most of these likely front-runners are out knocking on doors with the other legislative hopefuls in next Tuesday's election.

"Every single thing I can get to, I go to," said Del. Dorothy McDiarmid, a Democrat from the 18th District representing the northern half of the county, including Falls, Church and Fairfax City.

One of seven candidates for five spots on her party's ticket, McDiarmid, 72, and her fellow incumbent, Del. Kenneth Pulm, 37, are both interested in education and juvenile delinquency prevention issues. McDiarmid was first to the House in 1960, while Plum, a public schools administrator in the county, is running for his second term.

Most of the candidates in the 18th, a huge district with about 127,000 registered voters, are stressing the area's transportation problems. Attention has been focused on funding Metro and improving the roads in the western part of the county.

M.F. (Mike) Morris, 45 making his first political race, has followed a polite custom of the district by inviting all the Democratic primary contenders to his own campaign party "so my neighbors could meet them." A retired Air Force captain now working as a computer systems consultant, he is particularly concerned with energy problems and says Virginia has been slow to provide conservation incentives.

Dean Brundage, 55, an educational consultant and former administrator for the Consortium of Higher Education, also is making his first legislative race and hopes to work on legislation affecting education, government reform, conservation and agriculture.

Gary Eklund, 33, and Barbara Weiss, 48, have both been involved in local party politics for years. Eklund a sa es representative for a title insurance company, supports collective bargaining for public employes and says he would fight to keep abortions legal. Weiss, a writing and research assistant for the Government Finance and Research Center, is making her second try for a House seat and is a strong supporter of equal rights for women.

The seventh candidate, Wayne Lynch, 42, says he is running solely as an advocate of single-member legisative districts. A manager and planner for real estate investment and development, Lynch says he gives "the standard Democratic line" on other issues.

Five Republicans who thought themselves their party's nominees for the 18th District House race were suddenly thrust into a primary battle when a sixth candidate was allowed on the ballot after being initially disqualified.

"You tell me how anyone is going to campaign with just nine days left before the election?" complained Del. Martin Perper, one of two Republican incumbents, last week. Perper, 39, is completing his first term.

Del. Vincent F. Callahan Jr., 47 the other incumbent has managed to mail out fliers and do some telephoning to voters, but he said there is little sign of last-minute campaigning. He has served in the House since 1968.

The other Republican contenders include Helga Bonney, 36, a business-woman making her second House bid; John Buckley, 25, a policy consultant seeking public office for the first time; Jone Rust, 32, an attorney also making his first race; and Peter Scott, 45, a financial consultant who has run for public office twice before.

In the 19th District, the country's southern half, seven Democrats and nine Republicans are in separate primaries to select five party nominees each. The district has about 117,000 registered voters.

The Republican contest has three incumbents, Dels. Warren Barry, Robert Harris and Robert Thoburn, as well as James Dillard a high school government teacher. Dillard is a former House member who was successfully targeted for defeat two years ago by an ultraconservative GOP faction.

Barry, 45, has served in the House for 10 years. He describes himself as one of the few moderates among "Democrats who are very much left and liberal and Republicans who are very right and conservative."

Harris, 43, has served three terms and says his campaign in the 19th has centered around "pocketbook issues" such as taxes and the cost of living. He is running on a no-tax increase platform and would oppose any increase in the sales tax to fund Metro, as would most of the more conservative candidates in the race.

Thoburn, 50, was elected to the House two years ago and is regarded as Northern Virginia's most conservative legislator. Detractors in the paryt say he is "out of step' with the goals of the delegation, but others argue the assembly "ought to have more like him.

Thoburn, bragging that he has yet to make his first house call in any of his campaigns, said he expects to win reelection and is keeping his campaign strategy "a secret."

Dillard, 45, is a conservationist and says the current gasoline "crunch" makes the completion of the Metro system all the more important. He urges continued pushing for state and federal funding for the transit project.

Lawrence Pratt, 36, making his second attempt to win election to the House. A political consultant, he says Metro will not serve the county as well as buses, and he would like to crack down on juvenile crime. Executive director of the Gun Owners of America, he opposes the ERA and collective bargaining for public employes.

Belden Bell, 43, is deputy director of a study committee for 100 conservative members of Congress. He ran for Congress in Indiana two years ago and lost. An opponent of any special sales tax to fund Metro, Bell favors electing school board members and would limit the sales tax on food.

M. lee Minnis, 52, a retired officer who is now a real estate broker, says he would like to see a greater voter turnount among "the silent majority." He is also concerned that "Richmond is spending our money as though it was theirs."

John Adams, 49 owns a wallpaper and paint shop and is making his second House race. He describes himself as "a conservative in the strictest conservative sense," opposing the ERA, collective bargaining for public employes, abortion and the sales tax on food and nonprescription drugs.

Roger Grooms, 25, is a real estate agent making his first try for public office. He could not be reached for an interview, but said in a newspaper questionnaire that he opposes the ERA ratification extension and would favor a "standby special tax to pay for Metro operations.

The House Democratic primary in the 19th district attracted seven candidates, including incumbent Del. Gladys Keating, first elected in 1977. An ERA supporter, she said she is questioned more often about the sales tax on food but would "need to figure out where else to get money" if the tax were removed.

Lester Fettig, 31, quit his job as administrator with the White House office of federal procurement policy to make his first race for public office. A supporter of Metro, collective bargaining and the ERA, he would like to turn his management expertise to purchasing and utility rate concerns.

Charles Kaufman, 27, owns a direct mail advertising firm and has been active in local politics for several years. He says the county needs greater taxing flexibility, something he would work to secure in Richmond. He opposes taxing the elderly for food and non-prescription drugs.

Jane King, 38, has also long been active in local politics although this is her first campaign for public office. A teacher who has worked as a legislative assistant in Richmond, King wants to help "bring Northern Virginia's case to the assembly," especially its need for more funds for Metro and the county's roads.

David Temple, 31, is a school principal at a center for mildly retarded and emotionally disturbed children. Also making his first campaign for public office, Temple is the only black candidate among all the General Assembly hopefuls in Northern Virginia and has been active in county and state party politics. He said he would like to help alleviate the housing, education and utility cost frustrations of county residents.

Beverly Schwarz, 31, has a background in public administration and city planning and is making her first race for elective office. Arguing that "Vepco has taken the consumer over the coals," she would like to put a consumer representative on the State Corporation Commission and better define the agency's vote review procedures.

Williams Randels, 49, is a Defense Department consultant who owns a public relations firm and is making his first campaign for public office. He would freeze real estate taxes at initial appraisal values and would abolish state and local utility taxes. He favors earmarking all state alcohol and tobacco taxes for health and education needs, not roads or prisons.