When an ordinary citizen is accused of snorting cocaine, federal authorities usually don't even bother to investigate; the charge is only a misdemeanor.

But when such an allegation is made against a high government official, the situation is entirely different; the mere lodging of the charge triggers the special-prosecutor provisions of the new Ethics in Government Act and the FBI is automatically called in to investigate.

That's how Hamilton Jordan, the White House chief of staff, finds himself the subject of Justice Department investigation. Attorneys for two owners of New York's trendy Studio 54 disco claimed last week that Jordan had used cocaine during a visit there in 1978.

Jordan has denied the charge. He has been supported by Tim Kraft, a former presidential aide now running President Carter's reelection campaign, who yesterday told The New York Times he had accompanied Jordan to the disco on June 27, 1978, and was "sure" that Jordan had not used any illegal drug while there, or at any other time.

The White House has issued a statement noting that the accusers, Steve Rubell and Ian Schrager, are awaiting trial on tax evasion charges unrelated to drug dealing. The two men "have a clear interest in making false and sensational charges in an effort to bargain for leniency," a spokesman said.

A source close to the defendants countered that prosecutors had been pressuring the pair to cooperate in naming prominent people who used drugs in the nightclub.

The result is that Jordan's name is in newspaper headlines again and a team of FBI agents has fanned out to interview persons who may be able to verify or discredit the allegation.

This new investigation confirms the uneasiness with which many Justice Department officials view the special prosecutor provisions of the ethics act. They have been saying for months that the guidelines triggering such investigations are so loosely drawn that they invite abuse.

The law, which grew out of disclosures that the Justice Department was compromised at times during the Watergate investigation, requires an inquiry "when the attorney general receives specific information that a specified individual may have violated a federal criminal law..." After 90 days, the attorney general can decide the allegation was so frivolous it can be dropped, but if any doubts at all remain, he must ask a special court to appoint a special prosecutor.

Justice officials envision a time, especially with the approach of a presidential election year, that the allegations could become so numberous they disrupt the normal operations of the FBI and result in the appointment of a whole cluster of special prosecutors.

In interviews several weeks ago -- long before the allegation against Jordan arose -- Benjamin R. Civiletti, who since has become attorney general, and Philip B. Heymann, head of the criminal division, expressed their concerns about the new law.

Heymann said it would be possible for a political foe to file a complaint against a Carter administration official next year, timed so a special prosecutor might have to be named just before the election.

Another Justice official involved with policing the act said he felt sure it would have to be amended eventually.

Congressional drafters of the law countered that Justice officials were "crying wolf" about the dangers.

Civiletti acknowledged then that it was too early to recommend changes. But he said he wondered whether the law was too easy to trigger, too many people were covered, and the 90-day limit to investigate too short. He also suggested then that maybe the only crimes covered should be ones related to the officials' public duty.

Several investigations have been started under provisions of the act since it went into effect last fall. Until now the only case made public was one in which Carter White House officials were cleared of potential campaign law violations for allegedly soliciting campaign contributions during a luncheon at the White House.

The allegation against Jordan was first brought to the attention of Justice Department officials last Wednesday, officials said. Attorneys for Rubell and Schrager, meeting with prosecutors in New York the previous weekend, mentioned that they had information about an unnamed "senior White House official," source said.

Civiletti was briefed on the matter Thursday and called President Carter that evening to tell him he was formally starting the investigation, according to a Justice spokesman.

According to the allegation, first reported in yesterday's New York Times, Jordan visited Studio 54 in April 1978 and snorted cocaine in a secluded basement area. Alleged witnesses were Rubell, a reputed drug dealer known as "Johnny C" and Jody Powell, White House press secretary.

Powell said in a phone interview yesterday that he had never been to Studio 54 in his life. He added that he had been questioned by the FBI.

Jordan has been to Studio 54, with Tim Kraft, another presidential aide who just left the White House to run the Carter reelection campaign, sources said. But that visit was probably in late June, not April, they said.

Kraft, who also has been questioned by the FBI, said yesterday, in denying that Jordan had snorted cocaine during their June visit to the disco, that Jordan had been out of his sight "for no more than 10 or 15 minutes," according to The Times.

This is the second time Jordan has been accused of wrongdoing by persons facing criminal charges.

Last fall columnist, Jack Anderson reported that Jordan and Charles Kirbo, another Carter confidant, were linked to a $10 million deal to help fugitive financier Robert Vesco with his legal problems.

R. L. Herring, Georgia businessman convicted of fraud and racketeering charges, acknowledged heading the Vesco plot. And FBI investigation of potential obstruction of justice discovered no evidence that Jordan did anything improper.