A moderate Republican and two Democrats have found themselves seemingly united this fall in the face of a challenge by two conservative Republicans who are running a race that will test the impact of direct-mail politics in Northern Virginia.

Del. John R. Rust Jr., a 34-year-old Republican bitter that the two conservatives didn't include his name on a sample voting ballot during last month's GOP primary, has rejected overtures to campaign with fellow Republicans, Del. John S. Buckley and George C. Landrith Jr., for the three seats in the Virginia House of Delegates from northwest Fairfax County. "Each of us is running a separate campaign by choice," said Rust, a one-term incumbent. "It's unfortunate."

With only two Democrats running in the newly created 50th House District -- an area that includes Reston, Vienna and Herndon -- Rust has appeared to be more aligned on some issues with the moderate Democrats, Del. Dorothy S. McDiarmid, 74, the county's senior Democratic delegate, and former Del. Kenneth R. Plum, 34, than with the two Republicans.

Buckley, at 28 the youngest member of the General Assembly, and Landrith, 39, a consultant, are running a campaign that promises a repeat test of the impact of direct mail. A mass mailing conducted by New Right direct-mail wizard Richard Viguerie was credited in 1979 with giving Buckley, then a newcomer to suburban Washington politics, an upset victory in what was a 10-way race for the five House seats from the northern half of Fairfax. Buckley, cousin of conservative columnist William F. Buckley and a personal friend of Viguerie, says he plans a similar mailing this year.

Rust, formerly Fairfax city attorney, angered the conservatives during the primary by changing his position on the Equal Rights Amendment. When he first ran for the House in 1979 he opposed the measure that would ban discrimination on the basis of sex. Now he is backing ERA ratification by the Virginia General Assembly, where the amendment has remained bottled in a House committee for nine years.

"It was a hard decision to change my position on equal rights for women," said Rust. "But ultimately I don't know of anything that has as much priority as equality under law."

Both Landrith and Buckley oppose the Equal Rights Amendment, favor capital punishment, tax reform and locally elected school boards. Buckley, who has been rated as one of the most conservative delegates in the 100-member house, minces no words about his stance. "I'm a conservative who is in favor of limited government, free enterprise and individual liberty."

The two other Republicans seem untroubled by the division in the GOP slate. "There's a clear-cut choice this time," said Landrith. "People can really choose because we differ on just about all of the issues."

Democrat McDiarmid is stressing her 18 years of experience in the House. If reelected, she says, she will become chairman of the House Education Committee and the only Northern Virginian to head a major House committee.

She has shepherded a wide range of legislation in Richmond and claims credit for laws setting up public school kindergartens, establishing the George Mason University law school, enacting property tax relief for the poor and providing elderly and transportation assistance.

Democrat Plum, director of Fairfax County's adult education program, is seeking to return to Richmond. He lost two years ago after serving a single term in the House. He says he shares McDiarmid's concerns about social issues and supported legislation to increase funding to local governments for increased police protection and to change insurance laws to make mental health care more readily available.

Plum says he favors combating crime by insisting on restitution to victims by parents of convicted juveniles. In fast-growing western Fairfax County, party labels generally are said to be less important than in many other jurisdictions. Candidates in the district say that one out of every four voters they button-hole will most likely be a new resident.

Rust describes himself as a fiscal conservative who believes the state government can provide better services than the federal government. He supports capital punishment and says he would resolve Northern Virginia's road problems by supporting efforts to transfer general fund surpluses to the state's highway construction funds.