The special counsel of the Merit Systems Protection Board, in the first such action under the Reagan administration, has intervened on behalf of a career employe who says he was fired by Interior Secretary James G. Watt because he was a Carter Democrat.

Responds Lyn Nofziger, President Reagan's assistant for political affairs: "That's the best reason for firing anyone."

Special Counsel Alex Kozinski, a former Reagan White House aide, said yesterday that an investigation by his office has uncovered evidence bearing on the contention of John R. Gingles that he was fired from his National Park Service job because he was a Democrat once appointed by Carter to an Interior Department position.

A board hearing is scheduled Oct. 5 on Gingles' case, and Kozinski, who serves as a sort of special prosecutor and ombudsman for federal employes, said: "We plan to put on evidence at the trial."

The Gingles case is but a minor classic in the Washington book of politics, which is republished with a new jacket every four years. Since being fired in June, Gingles, 38 and a single parent who lives with his 13-year-old daughter in Vienna, has been unemployed. Before that, in the gravy days of the Carter administration, he had served as liaison to the House of Representatives for Interior Secretary Cecil D. Andrus.

In 1980, Gingles left the presidentially appointed Schedule C job and became a career appointee as deputy chief of congressional liaison for the National Park Service. The GS14 job paid $49,229 and offered job security since his former boss, Andrus, had decided to leave office whether or not Carter was reelected.

Last May 26, Park Service Director Russell E. Dickenson wrote Gingles that his job was being abolished and that "there are no other positions available to offer you."

Dickenson's letter explained: "This action is necessitated by a cutback in the number of positions and funds in the Office of Legislation."

One day later, the park service's legislation office received its "approved fiscal 1981 funding level" in a memo from its acting director, Ira G. Hutchison.

According to a copy of that memo obtained by The Washington Post, Gingles is listed by name, with his salary level fully approved. Another memo that month notes that the 26-person legislation office was being increased to 27 in fiscal 1981.

Gingles petitioned for reinstatement, contending that his firing was based "solely upon my partisan party affiliation and identification with the previous administration . . . ."

Kozinski's office investigated and, in a notice of intervention filed yesterday, associate special counsel Lynn R. Collins said the "investigation convinced the special counsel that the reduction in force appeared to have been improperly motivated."

The notice said officials of Kozinski's office had met last month with Dickenson and his aides and asked for Gingles' reinstatement. The request was refused.

"The special counsel is intervening in this case upon determining that the RIF reduction in force was improper and upon the refusal of the park service to voluntarily reinstate" Gingles, the notice said.

The notice of intervention was delivered by hand to Watt. The Interior Department promptly requested a postponement of further proceedings, and the case was scheduled to be heard Oct. 5 by a board representative.

Kozinski will argue that Gingles should be reinstated, and a representative of Watt will argue why he should not. The board can order Watt to reinstate Gingles with back pay. A spokesman for Watt said last night he had no information on the case and could not say what Watt's next step would be.

Meanwhile, back at the White House, veteran political operative Nofziger said he, too, does not know details of the affair. But he offered some unabashed analysis:

"I have no guilt feelings about this," Nofziger said. "There are many legitimate civil servants. But there are others . . . who don't share the president's views . . . who feel they have found a way to hold a job in this administration.

"Why should a guy who supported Jimmy Carter for three years be able to stick around now and screw up what Ronald Reagan is doing?"