President Reagan is scheduled to nominate Ann D. McLaughlin, a career public relations executive, as secretary of labor this afternoon at a White House ceremony, sources said yesterday.

If confirmed, McLaughlin will succeed William E. Brock III, who left the Cabinet post this week to head the presidential campaign of Senate Minority Leader Robert J. Dole (R-Kan.).

Neither organized labor nor conservative Republicans are expected to oppose the McLaughlin nomination.

A top AFL-CIO official said yesterday he does not believe that labor will oppose the nomination. "She virtually has no political baggage," he said.

The only reason for opposing her, he said, would be her complete lack of experience in the labor area. Therefore, he said, "at this point we wouldn't ask the {Senate Labor and Human Resources} committee to die on its sword to oppose her."

The AFL-CIO was notified yesterday of McLaughlin's pending nomination. A ranking federation official said labor might consider opposing her if Reagan's term of office were not almost finished.

An official of the Heritage Foundation, a conservative think tank with close ties to the administration, said he does not expect Republican opposition to McLaughlin. He predicted she will have support of the party's conservative and moderate wings.

"The moderates like Ann because she's the only one that can put John McLaughlin in his place," he said. She is married to television commentator John McLaughlin.

Ann McLaughlin served three years as assistant treasury secretary for public affairs in Reagan's first administration. Later, she was appointed undersecretary of the interior, a post she held for three years before resigning last March.

She also served as director of communications in President Richard M. Nixon's reelection campaign and later as director of communications for the Environmental Protection Agency.

Outside of government, McLaughlin, 45, has spent much of her career as a public relations executive.

Sources said yesterday that the two finalists for the labor post were McLaughlin and Constance J. Horner, director of the federal Office of Personnel Management. Horner would have been opposed by organized labor because of her dealings with government unions.

McLaughlin and Horner were seen by the White House as very much alike, sources familiar with the selection process said yesterday. "Both of them are considered to be good managers and decision-makers, and both would make good spokeswomen for the Republicans in the 1988 presidential election campign," one source said.

If confirmed, McLaughlin will find herself in an immediate fight with organized labor over such legislative issues as the federal minimum wage, plant closings, parental leave and child care.