Virginia State Del. Owen Pickett came under increasing pressure from Democratic party leaders yesterday to throw open the party's nominating convention, which until recently seemed certain to pick him to seek the seat being vacated by Sen. Harry F. Byrd Jr.

The movement for an open convention, begun during the weekend by a handful of liberal activists, reflects a growing number of complaints from party regulars that the conservative Pickett, an obscure state legislator backed by Gov. Charles S. Robb and others, will lead the party to a debacle in November.

"We're on a collision course that is threatening to destroy everything we built up with the Robb victory," said Paul Goldman, who helped form a committee backing an open convention. "There are hundreds of thousands of Democrats out there who want an alternative."

Several party leaders, openly worried about the threat of an independent Senate candidacy by state Sen. L. Douglas Wilder (D-Richmond), said privately yesterday they had urged the Virginia Beach delegate to release delegates pledged to him and allow the nominee to be chosen on the convention floor.

Most said they believe Pickett probably would still win because he already has enough delegates pledged to him to capture the nomination. But they said a public declaration opening the convention--creating the possibility of a draft movement for someone else--would be exactly the kind of bold, dramatic move that could breathe life into Pickett's faltering campaign.

Pickett campaign manager Tim Ridley said yesterday that an open convention is "something that has been considered" by the candidate and will be addressed at a press conference he has called for this morning in Richmond.

The push for an open convention is the latest embarrassment to plague a campaign that began with high hopes six weeks ago after Robb and other Democratic chieftains proclaimed Pickett as their consensus candidate to oppose Rep. Paul S. Trible, the certain Republican nominee.

The gravest threat to Pickett has been Wilder, the state's top black elected officeholder, who took offense when Pickett opened his campaign by invoking Byrd's name. That name is anathema to many Virginia blacks who vividly remember the days when the Byrd organization led the fight for "massive resistance" to school integration.

Pickett compounded his problems last week when Wilder and other black leaders took their complaints to top Democrats and Robb at a meeting in the governor's office. Pickett was "given every opportunity" then to renounce his comments about Byrd, according to one black at the meeting. Pickett, however, failed to address that question and instead reportedly talked about "keeping the party together" -- a posture that Wilder says has only fortified his intention to run.

Many Democrats say they also are concerned about Pickett's bland personality and his failure to ignite the party rank and file and campaign contributors during recent tours around the state. A 51-year-old Virginia Beach lawyer and certified public accountant known as a low-profile legislator, Pickett was chosen by Robb and other party leaders as their compromise choice because, they said, he was the candidate with the "least negatives."

Pickett's fund-raising, nonetheless, has been agonizingly slow. He reported a meager $40,000 in contributions, mostly from hometown supporters in his first Federal Elections Commission report last week.

When Pickett began showing up at party gatherings, such as a meeting of the Arlington Democratic County Committee this month, several supporters walked away shaking their heads over his performance. "I wouldn't characterize him as a boy orator," said Arlington County Board member John Milliken, in what many might view as an understatement.

"A lot of people are worried that he doesn't have the charisma or the dynamism that it takes," said State Sen. Clive DuVal (D-Fairfax.) "I'm worried that we haven't picked the best guy."

The key question, however, is what disgruntled Democrats can do to reverse the situation. None of the most talked about alternatives -- State Senate Majority Leader Hunter B. Andrews (D-Hampton), Lt. Gov. Richard J. Davis, Alexandria lobbyist Bill Thomas -- has actively promoted their own candidacies, saying little more than they would be willing to accept a convention draft.

Pickett has captured an estimated 1,942 of the 3,500 delegates to the Roanoke convention -- 182 more than the number needed to nominate.

"You have to remember that many of the people who have been selected are people who have already expressed an interest in supporting him," said Wilder yesterday. He said that an open convention "is not going to effect my decision to run.

"I think it's pretty well settled that whether I run or not, Owen has got his problems. I don't think anybody expected that the blandness and insipidity was going to be that bad."