SOUTH PORTLAND — National Semiconductor Corp. said Tuesday it would lay off 150 of the 1,800 workers at its plant.

Layoff notices are to be distributed in the next several months to the employees, who learned of the work force reduction Tuesday, said Mary Coady, spokeswoman for the electronics company.

The cuts, which will result in the elimination of 2,000 jobs company-wide, are part of a reorganization linked to reductions in federal defense spending, Coady said. National Semiconductor currently employs 32,000.

Most of those who will lose their jobs in South Portland work in a division that assembles electronic components for the Defense Department, Coady said.

She said layoffs generally will be determined not by seniority but by the employee’s job, skill level and past performance. Most of those laid off would not be people on the production line, she said.

Employment in National Semiconductor’s South Portland plant had increased modestly this year on the strength of the plant’s primary role as a maker of computer parts known as logic components. These are computer chips used in a wide range of computerized devices.

Many workers on the defense side have moved over to the logic business, but “there’s only so much the logic organization could be expected to absorb from the fall-off on the defense side,” Coady said.

Stephen J. Adams, an economist with the Maine State Planning Office, said companies nationwide that do work for the Defense Department are feeling the impact of budget cuts, “and obviously, Maine is not going to be immune from that.”

He said the number of manufacturing jobs in Maine has grown slightly in recent months, but the overall employment picture is not particularly encouraging because “New England seems to be leading the nation in this economic slowdown.”