Maryland Acting Gov. Blair Lee III, taking the offensive for the first time in his campaign for a full term, yesterday accused his closest Democratic challenger of "bartering away the financial security of the state" to win the endorsement of Maryland's largest labor organization.

Lee said Baltimore County Executive Theodore G. Venetoulis, who was endorsed Friday by the political arm of Maryland's AFL-CIO, has traded away "any hopes this state has to stop tax increases" by promising to support legislation that would require collective bargaining for public employes.

"I was not prepared to promise (the public employes' union) the sun, the moon and the stars because I could not in good conscience or in good sense mortgage the future of the Maryland taxpayer," he said, raising his voice loudly as he spoke at a press conference in Baltimore.

The convening of the press on a weekend afternoon and his denunciation of Venetoulis broke with Lee's practice heretofore of remaining above the campaign fray. Playing the role of the frontrunner, he has avoided drawing public attention to the campaigns of his opponents.

The AFL-CIO endorsement Friday was seen as a significant boost for Venetoulis, who has gained ground on Lee according to a recently published poll, The 350,000-member labor organization can fuel a campaign with thousands of volunteers and thousands of dollars in campaign donations.

Venetoulis, responding to Lee's charges at a hastily called press conference of his own, accused the acting governor of "insulting the working people of Maryland" by suggesting that te labor endorsement "was others by his own standards."

The Baltimore County executive said that he supports collective bargaining for public employes because all workers should have the legal right to a panel of their choice to negotiate their wages with management. Since successfully sponsoring a collective bargaining law in his county, he said taxes there have fallen.

Lee, who actively sought to block a labor endorsement for Venetoulis, said he supports collective bargaining for government workers on all matters other than wages. It makes good sense, Lee said, to give state employes the right of collective bargaining on their working conditions.

But once they have collective bargaining rights in wage talks, Lee said, public workers will insist on a strike clause or binding arbitration to give them leverage. A strike clause would be intolerable, he said, because it could lead to a serious disruption of vital services."