The primary defeats of two veteran Democratic Virginia legislators will force a partial realignment of power in the General Assembly, with Northern Virginia and the state's far southwestern area expected to gain the most.

The results also showed the increasing influence of black voters in deciding Democratic primary races, some political observers said.

In one of the most potentially significant shifts, state Del. Dorothy McDiarmid, 77, of Fairfax, who must first overcome Republican opposition in the fall, is now in line to take over the House Appropriations Committee. The committee oversees the state's $17 billion biennial budget and is considered one of the most powerful posts in the legislature.

"It's just a great day," said state Sen. Clive L. DuVal of Fairfax, chairman of the Northern Virginia delegation. "It will help safeguard the state Metro money. It'll help George Mason, Northern Virginia Community College and all our other educational institutions," he said.

The Appropriations chairmanship had been expected to go to 18-year veteran Democrat L. Cleaves Manning of Portsmouth, who was upset by a 32-year-old lawyer, Kenneth R. Melvin, in Tuesday's primary. Melvin, who is black, won by about 400 votes out of 8,000 cast in a race in which he said Manning had become out of touch with the district.

In a second major change, Democrat Claude W. Anderson of Buckingham County was defeated by Watkins Abbitt Jr., son of the area's former longtime congressman. Abbitt, 40, won with about 55 percent of the vote. The Anderson-Abbitt race had attracted widespread attention because of the bitterness of the contest and whisper campaigns directed at both candidates.

Abbitt faces Republican opposition from Cumberland attorney James McClellan.

Manning's defeat came as a surprise in a contest in which most of the area's black and white leaders either supported him or remained neutral.

DuVal said Northern Virginia legislators had considered Manning a friend -- the group sent him $1,500 -- but that there was no substitute for having your own legislator as head of a powerful committee.

"I'm not going to say today yippie, yippie, I'm going to hop into it," McDiarmid said today of the chairmanship. "I think that's a little soon."

McDiarmid said, however, that she is "obviously more interested" in the chairmanship of the powerful Appropriations Committee than in retaining chairmanship of the House Education Committee.

McDiarmid said the Appropriations chairman has "no little cache of money" to spend, but she noted that the position would give Northern Virginia more prestige and credibility. "The pictures of all the past chairman are on the back wall of the committee room and none of them is from Northern Virginia."

McDiarmid faces Republican Jean Morrison, a local GOP activist, in next fall's general election. McDiarmid has faced tough recent campaigns as her district has become increasingly Republican-dominated. Morrison could not be reached for comment.

DuVal, who also heads McDiarmid's campaign finance committee, said her campaign will make the pending Appropriations chairmanship "one of the key points in her brochures and statements -- she can do things for us no other delegate can do."

Manning had been expected to succeed Del. Richard M. Bagley of Hampton, who is retiring from the House to mount a 1989 campaign for governor.

Since Democrats control the assembly, House Speaker A.L. Philpott (D-Henry) will make committee assignments, but he usually goes by members' seniority.

Manning's district is 60 percent black and he attributed his loss to blacks supporting a black candidate. The Manning and Anderson primaries drew fewer than half the registered voters in each area.

"Fewer and fewer people are participating in Democratic primaries, particularly whites," said Larry J. Sabato, a University of Virginia political scientist. Sabato said the defeat of Anderson and Manning were local races that will "have no bearing on" statewide races this fall. But he said the contests indicated the increased importance of the black vote in deciding Virginia campaigns.