State Del. Owen Pickett, a little-known lawyer and accountant from Virginia Beach, launched his campaign for the U.S. Senate yesterday with an endorsement from Gov. Charles S. Robb and other leading Democrats.

Picket, 51, a soft-spoken legislator who served a year as chairman of the state Democratic Party, emerged last month as the favorite of party regulars to challenge U.S. Rep. Paul S. Trible (R-Newport News), the leading Republican contender in the race to succeed Sen. Harry F. Byrd (I-Va.).

But the unity that Democrats have tried so carefully to orchestrate over the last two months was challenged yesterday when one of the state's leading black politicians, angered by the failure of Robb and other Democrats to defend black issues during the recent legislative session, said he is seriously considering running against Pickett as an independent.

"There comes a time when people must be shown we don't have to tolerate pats on the shoulder," said Sen. L. Douglas Wilder (D-Richmond), the only black in the Virginia Senate and a key figure in Robb's gubernatorial campaign last year. "We'll show them. What we're going to say to those Democrats is 'Don't count on our support.' "

In lining up behind Pickett for the U.S. Senate nomination this year, Democrats hope to avoid a repeat of the fractious rivalries that plagued the party in recent years and helped produce a string of statewide Republican victories. The nominee will be formally selected in June at the party's convention in Roanoke.

Last year was the first time in 16 years that the Democrats, united behind the Robb ticket, were able to elect their own to the state's top three offices. This year, confronted with the unexpected announcement of Byrd's retirement, they labored to come up with the same winning formula: a moderate-to-conservative candidate whose lack of a controversial record could win broad-based support.

After a series of private meetings between potential candidates and party leaders, Pickett emerged as the candidate "with the least negatives," said one party member. His selection, given the tacit blessing of Robb, became nearly certain when three of his rivals--former state Attorney General Andrew Miller, former state Democratic Chairman William Thomas and state Sen. Edward Holland (D-Arlington)--took themselves out of the race.

Of the five serious candidates involved in the private selection process, Senate Majority Leader Hunter B. Andrews (D-Hampton) is the only one who has not withdrawn from the race. Today, Andrews, who ran an unsuccessful race for the Senate nomination in 1978, had no comment on his plans.

"We don't know of anyone else putting together a campaign," said Timothy Ridley, Pickett's campaign manager.

For Pickett, the Senate race will cap a quiet legislative career in the fast-growing suburbs of Virginia Beach, an area that, not coincidentally, is near Trible's Tidewater base of Newport News. In late 1980, Pickett was Robb's choice to succeed Richard Davis, now lieutenant governor, as state party chairman, a role that, his colleagues say, he fulfilled in a workmanlike way.

"He is reliable and predictable in the best sense of the word," said Party Secretary John Milliken. "You can divide Senate candidates into two categories: workhorse and showhorse. Owen will be a workhorse. He's not flashy or splashy. That's his reputation in the General Assembly and that's the way he handled the party chairmanship."

Pickett's work for the party was rewarded this fall when local party leaders, polled as part of the selection process, found him the best suited Senate nominee. "He is not so well known, but he is well-liked by those who do know him which is why it looks like he will have no opposition," said Bobby Watson, an aide to Davis.

Back home, Pickett, who was the top vote-getter last year in his multimember district, is known for his low-key manner. "When he campaigns, it's a soft, delicate approach," said Virginia Beach City Councilwoman Meyera Oberndorf, a Republican. "He's never generated any controvsery that would allow a contender to pit themselves against him."

As a legislator, Pickett's voting record has been generally conservative. Last year, during the debate on a proposed state holiday in memory of Martin Luther King, he voted to send the bill back to committee, which would have effectively killed the measure. This year, Pickett voted against two tax measures sought by Robb, including the 3 percent tax on wholesale gasoline sales, which helped finance additional money for Metro, and a revision of state corporate taxes pushed by the Virginia Education Association as the best vehicle for funding higher teacher salaries.

Pickett, a 10-year veteran of the General Assembly and a member of the powerful House Appropriations Committee, said yesterday he thought the state could have paid for higher teacher salaries by other means. The revision sought by Robb was, he said, "the wrong signal to send to the business community at this time."

In his kickoff speech yesterday, scheduled for delivery at eight stops on a two-day tour of the state, Pickett spoke of his "appreciation of Virginia's live-within-your-means fiscal traditions" and of the the state's "historic understanding of the importance of military strength"--themes Trible also emphasized last month when he began his campaign.

At a stop in Arlington yesterday, Pickett was surrounded by a dozen local Democratic officeholders and more than 100 party workers and supporters. His campaign staff, on the job for several weeks, said Pickett has already hired Washington-based media consultant Robert Squire, who managed the media campaign for Robb last year, and William Hamilton, a Chevy Chase-based pollster with experience in political races in the South.

In many ways, Pickett and Trible have begun their campaigns on parallel lines. Both waited for solid backing from their party's leadership before announcing and both are now trying to line up support from the state's major contributors for a race that is expected to cost more than $2 million each.

In the Trible camp, the recent appointment of White House aide Judy Peachee as campaign manager brought complaints from the GOP's right wing who question Peachee's conservative credentials. And now Wilder's threat to run as an independent raises question about the level of black support for Pickett, whose record on black issues, said Wilder, is "undistinguished."