The union representing federal people who print our money, take it back in taxes, monitor the airwaves and oversee national elections has gone to court charging that the first thing Ronald Reagan did when he became president was downright unconstitutional.

Attorneys for the National Treasury Employees Union (NTEU) have asked the U.S. District Court here to invalidate part of the federal hiring freeze that President Reagan ordered after he took the oath of office and made retroactive to Nov. 5, the day after he was elected.

If NTEU should win the class action lawsuit, the government could be forced to hire and give back to upwards of 20,000 persons who were promised jobs between Nov. 5 and noon of Jan. 20 when Reagan actually became presdient.

NETU represents white collar workers at the Internal Revenue Service, Bureau of Public Debt, Bureau of Engraving and Printing. Federal Communications Commission, Federal Election Commission, Nuclear Regulatory Commission and severl other federal agencies.

Reagan announced the hiring freeze moments after he became president. On Jan. 24 Dale McOmber, acting director of the Office of Management and Budget, issued freeze guidelines to agencies telling them not to put anyone promised a job after Nov. 5 on the payroll. Hundreds of workers hired after that date who were working when the back-dated freeze was announced will be allowed to keep their jobs. But many who had not started work have been told their job promises have been cancelled.

NTEU's lawsuit is filed on behalf of five individuals who were caught up by the freeze. One of them, Bernard A. Willer of St. Paul, Minn., reported for work Jan. 26 and was told, after four hours on the job, that his job promise had been negated by the OMB freeze memo.

Nobody knows how many people have been caught up in the freeze. In normal times federal agencies hire about 1,500 people per day. President Carter ordered a partial freeze last March. It was supposed to cut hiring in half. But many agencies have ignored his freeze in recent months and, since September, federal employment has been increasing.

The Office of Management and Budget is working up guidelines for agencies to process "hardship" cases -- people who quit jobs and moved or sold homes based on written job commitments from Uncle Sam. But decisions on granting exemptions to hardship personnel could be weeks, even months away.

Opponents of the freeze generally agree that Reagan had the power to stop hiring once he became president. But many argue that Reagan cannot legally reach back and invalidate job promises made before he became president. l

NTEU won what was considered a long-shot lawsuit years ago against then-president Nixon. He deferred a federal pay raise that was due in October until January. A federal court eventually ruled that while Nixon had the authority to defer the raise, he had not followed proper procedures. The government was forced to make retroactive payments to nearly a million white collar servants for that portion of the raise they had been denied.

In this case NTEU is asking that the court set the freeze date to Jan. 20, and order all persons promised jobs before that date put on the payroll with back pay.