A stalemate among top Virginia Democrats is expected to break today when Del. Owen Pickett, a Virginia Beach legislator and chairman of the state Democratic Party, signals his intention to run for the U.S. Senate, with the tacit blessing of Gov. Charles S. Robb.

The governor has avoided any direct endorsement, but according to various Democratic Party sources, he made clear at a meeting in the Governor's Mansion this week that he was encouraging the Tidewater legislator to become the first Democrat to formally seek the party's Senate nomination.

But the unanimity Robb has been seeking remains elusive as four other potential major candidates--former state attorney general Andrew P. Miller, lobbyist William Thomas, state Sen. Edward Holland (D-Arlington) and Senate Majority Leader Hunter B. Andrews (D-Hampton)--continue to consider the race.

Pickett, 51, will make a statement before the Democratic State Central Committee in Richmond today at which he is expected to announce his resignation as state party chairman. That's a first and necessary step if he is to run for the Senate seat being vacated by Sen. Harry F. Byrd Jr, an independent.

Pickett's announcement will come after weeks of private meetings among the leading contenders for the party's nomination, a process which ended last Tuesday with an after-dinner meeting with Robb at the mansion.

That process, and the stalemate it produced, already has prompted criticism from some Democrats who feel that the delegates to the party's nominating convention in Roanoke next June should be presented with a real, rather than an anointed, choice. "The lack of openness is a real problem," said Paul Goldman, a strategist from the Democrats' liberal wing who noted that the selection process has failed to focus on the candidates' stand on issues. "That's how you admit members to a country club, not to a political party."

The emergence of Pickett, a fifth-term delegate elected more than a year ago to head the state party with Robb's support, is proof of the effort to avoid controversy at all costs, some Democrats said. "Of course, he doesn't have any negatives because he hasn't done anything," said a supporter of another potential candidate.

For Robb, now the titular head of the party, and other Democrats, the critical issue is to get the Senate race under way. "If any one of these people had gone out for the nomination before this, they probably could have had it," said Del. Alson Smith, Jr (D-Winchester).

A Robb spokesman said this week that governor will have a statement on the Senate campaign next week. "The governor has always been interested in finding the most electable candidate," said Robb press secretary George Stoddart.

All five of the potential candidates this week declined comment publicly on the outcome of the meeting. A source close to one said Robb implied that he considered Pickett to be the strongest candidate although he was also "very reluctant" for anyone to conclude that Pickett was his personal choice.

"Pickett was given encouragement by the governor to proceed with his campaign, both structurally and financially," he said.

The desire for unity became more acute after Rep. Paul S. Trible of Newport News emerged as the all-but-certain nominee of the Republican party. "It is pretty clear that other party is probably going to avoid what can be fairly divisive process and a very strong argument can be made for doing the same on our side," said Robb in an interview two weeks ago.

To help identify the most electable candidate, Robb asked David Doak, his former campaign manager, to survey about 30 party leaders and members of the Democrats' traditional constituencies--labor, blacks and business--for their views. That survey concluded that Pickett had the broadest support. Several party officials said Larry Sabato, a political analysist at the University of Virginia, also advocated Pickett as the most electable of the contenders.