Skeptics expressed concern that the nominations would undermine the labor rights of the rising proportion of the work force that is nonunionized, in addition to those who are union members.

“The N.L.R.B. is a really important agency for unorganized workers,” said Catherine Ruckelshaus, general counsel of the National Employment Law Project, an advocacy group. “A lot of the workers we work on behalf of benefit from decisions the board has made under the Obama N.L.R.B., which are on the chopping block.”

The Senate might not consider Mr. Kaplan’s nomination until the fall.

Mr. Kaplan has spent a large portion of his career serving in political roles, including a stint as a counsel for Republicans on the House Committee on Education and the Workforce. The committee held hearings during his tenure scrutinizing prominent N.L.R.B. actions in which the witnesses skewed toward business representatives and other skeptics.

Mr. Kaplan also had a hand in legislation hemming in the labor board, most prominently the Workforce Democracy and Fairness Act, which would have effectively undone the board’s rules expediting election timetables. The legislation has yet to pass Congress.

“I found him to be very thoughtful and careful,” said Marshall B. Babson, who was a Democratic member of the board under President Ronald Reagan, and has worked for years as a management-side lawyer and interacted with Mr. Kaplan. “But he was in a much more partisan role in that position and he made no bones about it.”

Mr. Babson pointed out, however, that it is hardly unprecedented for a congressional aide to ascend to the board.

Whatever Mr. Kaplan’s appetite for politics, his record appears to place him in the mainstream of Republican labor policy. If Mr. Trump’s second board nominee fits the same profile — and the management-side lawyer said to be in line for the position, William J. Emanuel, appears to do so — they would almost certainly join with the board’s current Republican member, Philip A. Miscimarra, to undo crucial portions of the board’s legacy from the Obama era.