President Donald Trump tapped attorney Marvin Kaplan to fill a vacant seat on the National Labor Relations Board, taking a first step toward securing Republican control of the agency refereeing disputes between unions and business for the first time in nearly a decade.

The president announced his intent to nominate Mr. Kaplan, counsel at the independent Occupational Safety and Health Review Commission, to one of the five seats on the board on Monday evening. If he wins Senate confirmation, Mr. Kaplan would be the second Republican on the panel. A fifth position remains vacant, but it is expected the White House will name another Republican for that spot.

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Mr. Kaplan’s political background is likely to raise questions, said William Gould, a Democrat who served as board chairmen during President Bill Clinton’s administration.

“We need less politicization of the agency, and the Kaplan pick moves us in the wrong direction,” said Mr. Gould, a former Stanford Law School professor. The board, created in the 1930s, was intended to be a panel of experts appointed to adjudicate a complex law, the National Labor Relations Act. Placing political partisans from either side on it weakens that mission, he said.

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Mr. Gould said a longtime practicing labor-law attorney like Mr. Emanuel would be a less contentious pick. Mr. Emanuel is “very far to the right, but he’s well qualified,” Mr. Gould said.