A group of influential Virginia labor leaders, angered by Lt. Gov. Charles S. Robb's conservative campaign for governor, are moving to deny him the state AFL-CIO's endorsement, an action that could make Robb the first Democrat running for major statewide office in a decade to be spurned by organized labor.

Barring major political concessions by Robb before the AFL-CIO's state convention three weeks from now, leaders from a half dozen large unions say they have the votes to block Robb's endorsement.

Losing organized labor's backing would be a blow to Robb, whose campaign strategy revolves around holding the Democratic Party's traditional urban-labor-liberal base while attracting conservatives wary of GOP candidate J. Marshall Coleman.

Thanks to his strong probusiness stands supporting Virginia's right-to-work law and condemning collective bargaining for public employes, Robb has managed to woo many Virginia conservatives. But the opposition of some large unions in the state indicates that Robb's conservative support may carry a heavy price.

"As things look now, it's virtually impossible for him to get the endorsement," said 'thomas McNutt, president of Local 400 of the United Ford and Commercial Workers, which claims 12,000 Virginia members. "They [the Robb campaign] have pretty much taken the attitude they don't need us. For us to support him now telegraphs a message around the country that our endorsement comes pretty cheap."

Robb spokesman deny that the union support is unwanted, but the labor leaders, who also include representatives of the steel workers, telephone workers, public employes and machinists unions, have not been placated. They were incensed by a letter Robb wrote last April to state Sen. Elmon Gray, a Southside Virginia conservative, pledging to veto any legislation to weaken either the right-to-work or antibargaining laws affecting public workers. The letter has been used by Robb fund-raising operatives seeking money from conservative businessmen who in recent years have generally supported Republicans.

"To our mind, that letter was part of a deliberate and calculated assessment by Robb and his campaign staff that he can gain more votes by labor-baiting than he'll lose," said Robert Stewart, executive director of Council 30 of the American Federation of State, County and Municipal Employees, which has 3,000 state members.

The AFL-CIO leaders were also upset by Robb's statement, as he emerged from a meeting with President Reagan two weeks ago, of general support for the goals of the president's budget-cutting program. "If his father-in-law [the late president Lyndon Johnson] were alive, he'd be beating Robb over the head," Stewart said.

Robb campaign manager David Doak, who puts Robb's chances at winning the AFL-CIO endorsement at "probably less than 50-50," has personally taken charge of a last-ditch effort to win the labor leaders back before the convention. Doak has set up a series of meeting with the officials and says Robb will meet with them as well if any progress is made.

"We have our differences with labor but we want to talk about it," says Doak. "There's no question labor is an important part of the coalition we need to win, and we're not out to exclude them or anyone else."

But Doak said there are no plans for Robb to change or tone down his positions on issues, and the labor leaders say anything less is unacceptable.

Every Virginia Democratic candidate for governor or U.S. senator since 1970 has won the endorsement of the state AFL-CIO, which claims about 100,000 state members. While politically weaker than in most northern industrial states, the state organization and its 500 union locals have provided an important pool of campaign foot soldiers as well as money to Democratic candidates.

In the 1977 governor's race, labor unions accounted for more than $220,000, or 31 percent, of the funds raised in the general election by losing Democrat Henry E. Howell, The Norfolk Populist.

Robb has long cultivatedd the support of state AFL-CIO President Julian Carper, a soft-spoken moderate who was a Robb delegate to the Democratic state convention in May.But the Robb critics, who contend that Carper did to win any concessions from Robb in return for his support, say they control close to half the delegates at next month's convention -- more than enough to block the endorsement, which requires a two-thirds vote.

Robb also faces some unexpected problems in winning support of the Virginia Education Association, which represents about 44,000 state teachers. VEA President Walter Mika said he supports Robb, but added "a lot of people were rather upset to say the least at the antilabor atmosphere at the Democratic convention," where the letter to Gray was distributed and where Ira Lechner, an Arlington liberal, was defeated in his bid to the the party's nominee for lieutenant governor. VEA officials predict a close vote when the association meets July 26 in Farmville.

The Coleman campaign, which also has concentrated on the right this year, expects to get neither organization's endorsement, though it hopes to win some members' votes in November. Campaign manager Anson Franklin called Robb's labor troubles "a sign of an identify crisis -- he's spent so much time trying to court the conservative vote that he's spread himself too thin."