Ronald Reagan's transition talent scouts have filled the memory banks of two Texas Instruments computers with the names of qualified Republican job-seekers. But there's one Republican whose name the transition team would rather not hear these day: Mahlon M. Delong.

Delong is a GOP loyalist from Maine who held a good job in the federal Farmers Home Administration until 1977, when Jimmy Carter's transition team picked a Democrat to replace him. Carter's people said this was standard political operating procedure. Delong said it was wrong, so did a federal judge, who ruled this fall that Delong is entitled to have his old job back.

The Delong decision, and the Supreme Court decision last March on which it was based, have cast a legal cloud of uncertain dimensions over the president-elect's power to put his own appointees in thousands of federal jobs that have traditionally changed with a change of administrations.

Officially, the Reagan people say they are not worried about their legal authority to fire Carter administration officials now holding "plum" jobs in the federal bureaucracy. But they are concerned that some current officeholders might file suits like Delong's and thus wrap miles of legal red tape around Reagan's efforts to transform the government.

The federal Office of Personnel Management also expects some Carter appointees to go to court to keep their jobs. "I would be surprised not to see some," says Deputy General Counsel Paul Trause. "I don't expect to be deluged, but I think it's a real consideration."

The consideration goes back four years to a Supreme Court decision that it was unconstitutional for Chicago's Democratic machine to fire people who do not support the Democratic Party. That decision dealt with low-level clerical workers and was not thought to have much application to senior jobs at any level of government.

Then last March the court upped the ante. It ruled the underlying rationale of the Chicago case -- that the First Amndment protects goernment employes from retribution because of their private affiliation -- applied to certain government lawyers and, potentially, other fairly important positions. That case dealt with county employes, but Justice Lewis F. Powell, in a dissent, warned that it might also cover federal officials.

The decision, Powell said, "may cast serious doubt on the properiety of dismissing United States attorneys, as well as thousands of other policymaking employees . . . because of their membership in a national political party . . . Federal judges will now be the final arbiters as to who . . . governments may employ."

Powell turned out to be prophetic, at least in regard to Mahlon M. Delong's employment. Delong had been, under Republican Gerald Ford, the Maine state director of the Farmers Home Administration. When Carter took the White House, his transition team found Delong's job listed in "The Plum Book," the roster of jobs excepted from civil service rules. The new administration picked a new state director to replace him.

Delong sued to keep his job. He lost in the first round, but his appeal was pending when last March's Supreme Court opinion came down. On a remand, U.S. District Court Judge Albert V. Bryan Jr. decided that Delong was illegally fired and ordered the administration to make him the Maine state director again.

Neither the government nor Delong's lawyers want to speculate on how many other "plum book" jobs might now be beyond the reach of an incoming president as a result of the Delong case. Both sides note that Delong's job was a "Schedule A" position, a somewhat different species, legally, from most of the jobs in the book.

But the Reagan transition people are still concerned that some Carter appointees in the more common "schedule C" jobs might to go to court to argue that they, too, have First Amendment job protection. The Reaganites contend that they would win such suits -- the Supreme Court has exempted from its protection jobs that involve policy formulation or confidential relationships with elected officials -- but they also say legal actions could seriously delay the transition at some agencies.