Maryland's largest labor organization gave its unanimous endorsement to gubernatorial candidate Theodore G. Venetoulis yesterday, passing over a major Democratic incumbent for the first time in recent history.

The endorsement from the political arm of Maryland's ALF-CIO is expected to give new impetus to Venetoulis' uphill battle in the Democratic primary race against Acting Gov. Blair Lee III, the clear front-runner, and candidates Harry R. Hughes and Walter S. Orlinsky.

Thousands of labor volunteers will join Venetoulis' campaign from the lists of 350,000 state AFL-CIO members, and thousands of dollars in labor campaign contributions will be made available to Venetoulis and his running mate, Ann C. Stockett, as a result of the endorsement.

Maryland's AFL-CIO Committee on Political Education has traditionally backed incumbents as long as it approves of the candidate's labor record. It once even chose Republican Sen. Charles McC. Mathias over his Democratic challenger, now Rep. Barbara A. Mikulski. When COPE wants to register displeasure or when it finds the candidates have equally good labor records, it makes no endorsement, as in the 1974 gubernatorial race.

Lee had tried hard to persuade the group to endorse no one, and even threw a last minute party for labor leaders aboard the state yacht Saturday night. Before the vote yesterday afternoon. Lee's running mate Steny H. Hoyer went to the union hall to tell supporters that it would be best to concede to Venetoulis' obvious majority.

As he was ushered up the smoky aisles of Baltimore's Fire Fighter Union Hall. Venetoulis received a standing ovation from the more than 300 delegates to the endorsement meeting.

"I can't tell you what I feel today," he told the crowd. "It's a gutsy and a courageous thing you've done. You've stood up to the establishment and gone with an underdog . . . This is not an election we'll be going through, but a rebellion against the old line of power brokers and policial machines."

While a nod from organized labor may seem incongruous with his campaign for a "New Maryland", Venetoulis later defended his rhetoric by saying that, for him, the COPE endorsement means that "one more organization representing working people has joined me . . . I needed this momentum."

Earlier this summer, Venetoulis won the endorsement of the 35,000-member Maryland Education Association.

Joining Ventoulis at the union hall was Democratic attorney general candidate Steve Sachs, a former U.S. attorney for Maryland, who also won COPE's unanimous endorsement.

After the ovation, Venetoulis and Sachs the two victors after more than a year of campaigning to win the labor endorsement, clasped their hands together in a salute.

Sachs said later that "It means a lot to me that the Baltimore Building Trades Council was part of this endorsement. I prosecuted and convicted their former president (Guido Ioziz) 10 years ago and today, the man who replaced Ioziz helped give me this endorsement."

Labor officials publicly pointed to Lee's ambiguous record on labor issues as their reason for snubbing the incumbent. Privately, they cited numerous occasions, when, they say, they have found Lee's condescending air too much to bear. "He has made some outrageous off-the-cuff remarks against the labor movement, at times where it made no sense," said one high-ranking Baltimore labor leader.

Venetoulis, as Baltimore County Executive, has kept close ties with labor leaders in that area and received the strong lobbying support of the Baltimore AFL-CIO Council president Thomas Bradley and of Ernie Crofoot, from Baltimore County's chapter of the American Federation of State, County and Municipal Employes, the largest voting bloc in Maryland's AFL-CIO.

Crofoot lobbied for Baltimore County's first employe collective bargaining bill and in his acceptance speech yesterday, Venetoulis promised that he would push for a collective bargaining bill for the 110,000 state employes.

"My economic development plan for a new Maryland will give more jobs, secure jobs, to people in our state but not like in Mississippi where new jobs mean lower wages," he told the union members.

About a week ago, Maryland's statewide chapter of AFSCME voted to endorse Venetoulis and Stockett. During the last General Assembly session, Lee won the displeasure of state employes by promoting state employes pension bill they thought unfair. Lee has since made substantial modifications in his position.

In recent weeks, Washington area union leaders had been split between Lee and Venetoulis and some had tried to block Ventoulis in the state AFL-C10 executive board endorsement meeting that preceded yesterday's final endorsement vote.

Out of the 30 members of that executive board, only one voted for no endorsement at the private session, according to labor leaders. The one vote was from Joseph F. Curtice who resigned on the spot and has joined Lee's campaign.

Some Baltimore labor supporters of Orlinsky had also attempted to block Venetoulis' endorsement arguing that Orlinsky, as well as Hughes and Lee, had labor records that were too impressive to be overlooked.

Earlier this week Lee and Hoyer picked up the endorsement of Maryland's United Auto Workers, considered one of the most liberal labor groups along with AFSCME. In Maryland there are some 17,000 UAW members. Lee's ticket also picked up the endorsement of the Classified Municipal Employees of Baltimore.