When Virginia State Sen. L. Douglas Wilder of Richmond holds a fund-raiser Thursday night at a prestigious Alexandria law firm, it will be a measure of how far the black lawyer has moved toward becoming the Democratic nominee for lieutenant governor next year.

But when one of the key sponsors of the event -- Sen. Clive L. DuVal of Fairfax, the chairman of the Northern Virginia legislative caucus -- says he's going only out of friendship and that it "in no way connotes an endorsement," it's a measure of just how far Wilder has to go.

After last week's elections, in which the state's white voters overwhelmingly supported President Reagan and Republican Sen. John W. Warner, Virginia Democrats have nervously begun reassessing the impact of a Wilder nomination, troubled by what some say it would do to the races for governor and state attorney general.

Also troubling others is the drubbing that the party's first woman candidate for the Senate, Edythe Harrison of Norfolk, took in her race against Warner. Some Democrats are saying that could spell trouble for the party's only announced Democratic candidate for state attorney general, Patrick County Del. Mary Sue Terry.

"There are a lot of people in the party who just say that that ticket isn't going to win, and that's why I guess some of them started calling me," said state Del. Frank M. Slayton (D-Halifax), who said he is considering opposing Terry.

Slayton said today that all three of the party's candidates for governor -- Lt. Gov. Richard J. Davis, Attorney General Gerald L. Baliles and Hampton Del. Richard M. Bagley -- have encouraged him to run. Spokesmen for all three campaigns said today that their candidates have not endorsed Slayton and are less concerned about Terry being on the party ticket than Wilder.

"For better or worse, this is still Virginia," said George M. Stoddart, Gov. Charles S. Robb's press secretary. "That makes it very difficult. Once you get beyond the state Senate or maybe a congressional district, it becomes very difficult for black candidates."

A half-dozen Republicans -- including a black businessman -- are lining up to run against Wilder, who is the only annnounced Democratic candidate.

Virginia is one of only two states (New Jersey is the other) with statewide races in 1985. Because Virginia is a southern state, the elections next fall will be viewed as a test of the Democrats' ability to maintain strong black support and avoid the continued flight of whites into the Republican Party.

The racial issue is so sensitive that some top Democrats won't discuss it. "You're talking about the problem [facing Democrats] across the South," said DuVal. "I'm not ready to discuss it."

Blacks make up less than 20 percent of Virginia's registered voters, but because of their concentration in the Democratic Party they are believed to represent about 30 percent of that party's constituency and play a key role in its nominating processes.

The Democrats will begin selecting delegates for the summer nominating convention at mass meetings beginning in either late March or early April. This nominating process tends to involve party regulars and therefore some Democrats say it may give an advantage to candidates who are popular among party activists.

Some Democrats say Wilder, 53, who has been a controversial figure in Richmond politics, may be vulnerable for other than racial reasons. But they also express private concerns that any concerted move against him could alienate key black supporters, leaving the party in worse shape.

Democrats are eager to repeat 1981 when they swept all three state elective offices, electing Robb as the first Democrat chosen for that office in 16 years. Robb cannot succeed himself and there is a fierce fight within the party for the nomination by Davis, Baliles and Bagley. The candidates are weighing, and in some cases conducting polls to assess, the racial issue.

Representatives of all three campaigns and Robb have praised Wilder, an influential moderate for 15 years in the state Senate who is best known for his successful efforts to establish a state holiday for Martin Luther King Jr., the slain civil rights leader.

But the leaders also have expressed private concerns that the racial issue in largely conservative Virginia could bring down Wilder and the entire ticket. "That's their problem," said Wilder, who announced in July and said he would do his own polls on the issue.

"I have no intention of conducting a campaign as a black candidate," Wilder said. "I am not a black candidate. I am a candidate who incidentally happens to be a man, who incidentally happens to be black."

Wilder said he is drawing white support and interest and knows he cannot win without it. He said his treasurer is state Sen. Edward M. Holland of Arlington, and that other sponsors of his Northern Virginia fund-raiser include Alexandria Mayor Charles Beatley, Dels. Bernard S. Cohen and Marion Van Landingham, and Arlington County Board member John Milliken as well as other local officials.

The event is being held at the law firm of William Thomas, an influential lobbyist and Robb's friend. Wilder said Thomas has taken no position on his campaign and that a Thomas partner, Carl T. Rowan Jr., is hosting the event.

Party members differ sharply over Terry's candidacy. A seven-year member of the legislature, she already has drawn broad support from key legislative leaders and party fund-raisers, two groups that never supported Harrison.

Terry, 37, who would become the first woman to run for Virginia attorney general, said she hoped "voters' decisions would be made based on who is the best person for the job." She said she would not try to influence how the rest of the ticket is put together but would support the party's nominees.