The Carter administration has managed to overcome its own scruples and the legal obstacles created by the president's civil service reform act and is setting up a patronage-as-usual program for filling the 275,000 temporary jobs involved in taking the 1980 census.

Armed with an unpublicized memo from President Carter exempting the jobs from civil service, a veteran labor union political strategist and his two assistants, a black woman and a Hispanic, have opened a recruiting office to fill the census jobs.

Rex Granum, the deputy White House press secretary, said yesterday that Carter overcame his initial reluctance to approve the patronage hiring because "career people at the Census Bureau advised him...this was the only way to get large numbers of motivated people to take thse short-term jobs. Their experience was that hiring them through the civil service system would be entirely too cumbersome."

Mikel Miller, administrative assistant to Glenn Watts, the pro-Carter president of the Communications Workers of America (AFL-CIO), quit that job in June to become director of recruiting for the 1980 census.

His two special assistants are Shirley Robinson Hall of Detroit, a member of the Democratic National Committee since 1972 and until recently manager of civic affairs for a Michigan utility company, and Louis Delagdo of Houston, who was special assistant to Immigration and Naturalization Service Director Leonel Castillo.

The jobs they will hand out range from those of$4-an-hour enumerators, who go door-to-door, to to about 400 district office managerships, paying about $400 a week.

Some of the supervisory jobs will last about nine months, but most of the low-level jobs are only a few weeks in duration.

Historically, the temporary census jobs have been available as patronage to the party in power, but the Carter administration late last year discovered with some chagrin that it had unintentionally denied itself that privilege.

The civil service revision legislation sponsored by the president and passed by Congress last year contained no exemption for the census jobs.

That was learned by the White House last December only after Carter personally had persuaded Patti Knox, a Michigan Democratic leader and early supporter of his, to quit her job in Detroit and move to Washington to run the Census Bureau recruitment operation. It was Knox who discovered and informed the White House that Carter's cherished civil service reform had rendered the operation illegal.

Knox, who is not back in Detroit and in another job, said the White House at that time considered and rejected asking Congress to amend the law to exempt the census jobs or having the president exercise his authority, under the law, to exempt them for reasons of "good administration."

"I didn't think it would be the best thing to do," she said," after all the publicity that had been given to the passage of the civil service reform law."

But on March 26, Carter signed a memorandum providing the waiver from the general ban on giving preference to people referred by outside organizations or taking into account the political affiliation of applicants.

He said his goal was to "open up our recruiting to as many sources as possible in order to insure that this census contains the most accurate count of our population. I am particularly concerned that we draw as many qualified census employes as possible from the neighborhoods in which the census is being done."

Black and Hispanic organizations have expressed great concern about an "undercount" of poor and transient people. Miller said that in choosing Hall and Delgado as his recruiting assistants, "I felt we'd demonstrate our commitment from the top down to having people from these communities" involved in the count.

Miller said he would seek referrals from national and local organizations, including branches of the Democratic party, and minority organizations, from members of Congress and state and local elected officials and the White House itself.

"Preference will be given to Democrats," he said, "which is consistent with the hiring policies since the start of the census" that have made these jobs part of the patronage pool for the party in power.

Census employes are barred from partisan political activity during the period of their employment, but in most cases, including Miller's own, that ban will end several months before the 1980 election.