TUESDAY'S ELECTIONS in Virginia include

contests for the 100-member House of Delegates. These are important decisions for voters in this area, affecting their representation in the state legislature immediately as well as in the future, since these lawmakers will draw up a new redistricting plan for the state.

In Alexandria's 21st District, the two incumbents --Democrat Bernard S. Cohen and Republican David G. Speck--have served their constituents as well as all of Northern Virginia with intelligence and energy. Their quickness to grasp the complexities of budgeting and to build coalitions that work to the benefit of Northern Virginia have been impressive.

In Arlington County's 22nd District, where there are three seats to be filled, voters have an important stake in re-electing the three incumbents--not only because they have worked effectively in Richmond, but also because they have earned the respect of key lawmakers from other sections of the state. Democrats James F. Almand, Mary A. Marshall and Warren G. Stambaugh make up a team that works smoothly on behalf of the county.

In the Fairfax County/Falls Church 49th District, incumbent Vincent F. Callahan Jr. brings expertise and seniority to the job as a respected member of the House Appropriations Committee and a legislator who is sensitive to the need for areawide teamwork by Northern Virginia's delegation in Richmond. Democrat Elaine A. Lailas has waged a vigorous and well-informed campaign, raising a broad range of issues and winning the support of many area organizations. For the third seat, both Democrats Flora M. Crater and Conrad J. Marshall offer voters political experience that could be put to good use in Richmond.

In the Fairfax County/Fairfax City 50th District, one-term Republican incumbents John S. Buckley and John H. Rust Jr., 18-year veteran Democrat Dorothy S. McDiarmid and two challengers are competing. Mrs. McDiarmid's seniority in the next General Assembly could prove invaluable to Northern Virginians on issues such as the budget, redistricting and transportation. Mr. Rust has been quick to forge useful working partnerships with legislative colleagues from elsewhere around the state and also deserves re-election. For the third seat, the district can do better than it has by electing someone other than Mr. Buckley, whose overemphasis on heavy-handed budget and tax-cutting has shown little sensitivity to the need for regional cooperation on key financial questions involving Northern Virginia. Democrat Kenneth R. Plum, a former state delegate, has the necessary knowledge and constituent contacts to do better.

In Fairfax County's 51st District, where there are three House seats up for election, Republican incumbent James H. (Jim) Dillard II appears to have enjoyed strong constituent support during his performance in Richmond, which includes service as the ranking Republican on the House Education Committee. Neither of the other two incumbents, Republicans Lawrence D. (Larry) Pratt or Robert E. (Bob) Harris, has served with much distinction on the Northern Virginia delegation. One excellent replacement would be Democrat Vivian E. Watts. Her vast experience as a congressional legislative assistant, as a former president of the Fairfax League of Women Voters and as a past director of research and legislation for the Fairfax Chamber of Commerce, could be put to excellent use on behalf of the district and the region.

In Fairfax County's 52nd District, incumbent Republican Warren E. Barry holds important committee assignments in Richmond, including House minority whip. His 12 years of service in the House have been sensitive and intelligent. Similarly, incumbent Democrat Gladys B. Keating has demonstrated a keen sense of her constituents' problems and an understanding of regional-state relationships. For the third seat, Democrat Brendan P. O'Hara, who has been a legislative assistant on Capitol Hill, has demonstrated a good grasp of the issues.

In Loudoun County's 17th District, Democratic incumbent Earl E. Bell and Republican Kenneth B. Rollins, the mayor of Leesburg since 1978, have been locked in a vigorous contest. Both men have strong credentials in the district; on the basis of who is best equipped to address state budget questions, Mr. Bell, as the incumbent, enjoys an edge.

In the 23rd District (Prince William/Manassas/Manassas Park), none of the challengers has presented a case for unseating incumbent Democrats Floyd C. Bagley and David G. Brickley. Republican Harry J. Parrish, a Manassas town council member for 12 years, recently resigned mayor and a former president of the Virginia Municipal League, offers good experience for the third seat.