The sudden resignation of a top investigator, an accompanying dose of political rhetoric and some imprecise media accounts have fed what seems to be growing public impression that the South Korean government's effort to buy influence on Capitol Hill involves more than 100 members, a cover-up, indeed a new Watergate.

But a more detached look at the complexities of the case and the different investigate standards by which the personal conduct of members will be judged shows that such recent expectations may be inflated.

Any realistic assessment of the prospects of the current Justice Department and House committee investigations of the South Korean lobbying campaign demands an understanding of important differences in the twin inquiries.

The Justice Department, on one hand, is ivestigating possible bribery and conspiracy and tax evasion cases against public officials who took cash or gifts from agents of the South Korean government.

The charges are difficult to prove in any case. And the more than year-old Justice inquiry has been hampered as well by difficulties in tracing cash payments to members and proving that the recipients did anything in return or had knowledge the giver was a foreign agent.

In addition, the criminal investigation has been hurt by what critics say is its own ineptitude. A key witness, Tongsun Park, a Washington businessman who made cash payments to several members of Congress, was allowed to leave the country last fall. Obvious sources of pertinent records were overlooked for months. Grand jury witnesses have remarked they weren't asked the right questions.

As a result, the Justice Department attorneys are believed to be focusing only on a handful of congressional and a few Korean targets for possible indictments.

Justice officials wince when they see press accounts that estimate that as many as 115 members of the House "have been touched by the Korean scandal" and are being looked at by the separate House investigation.

The problem, in their view, is that the numbers get attached to their inquiry as well, leaving the impression that scores of indictments will be handed down by the federal grand jury now hearing their evidence in Washington.

Benjamin R. Civiletti, head of the Justice Department's Criminal Division, said last week that comparing the two investigations was like comparing apples and oranges.

While Justice is looking for clear-cut criminal violations of law, members of the House Committee on Standards of Official Conduct will be judging their peers by a harsher standard of post-Watergate ethics and appearances.

Indeed, the House committee has adopted a "manual of offenses" that makes it likely that many members could face disciplining for unethical conduct, even if their actions can't be proved criminally wrong in a court-room.

Expectations for the House inquiry have been heightened, too, by a series of highly publicized developments over the past two weeks.

First came a New York Times report that 115 members were "involved." That was followed by the sudden resignation of Phillip A. Lacovara, the committee's speical counsel, after a dispute about the pace of the investigation.

Repbublican leaders renewed calls for a Watergate-style special prosecutor. Whispers of cover-up by junior House members were fueled by an ABC-TV report, since discredited, that implied the House leadership was implicated in the Korean scandal.

Finally a big-name attorney, Leon Jaworski, the special prosecutor hero of Watergate, was brought in to calm the storm. But that just gave the Watergate analogy further credence, especially when he started right off talking about prosecutions - which is Justice's job, not his.

The "manual of offenses" approved by the House committee does open the way for holding members liable for standards of conduct much stricter than those judged by Jaworski's former Watergate lawyers or Civiletti's current Justice Department Korean investigators.

The manual states that members of Congress are "expectec to adhere to standards of conduct far more demanding than the bare minimum standards established by our criminal law. In order to protect the integrity of our government process, public officials must take care to avoid even the appearance of impropriety."

Thus, the House committee uses a standard which makes colleagues liable for disciplinary action if they neglected to ask the purpose of a gift from the South Koreans. Most criminal statues require that the person hve knowledge that the gift was meant to influence him.

Likewise, the committee established a burden of proof less exacting than that required by criminal law.

Some of the few members of the House who have noticed the strict standards adopted by the committee complain privately that the rules of the game have been changed.

The Constitution says public officials shall accept no "present, emolument, office or title of any kind whatever from any king, prince or foreign state."

For years, some members of Congress have accepted trips fro foreign governments or thinly disguised front groups. But the fact is the House never has been anxious to police its own standards of conduct.

Now that the committee has adopted a new standard for judging such conduct, the manual says that punishment for violating the constitutional prohibition should depend on the value of such a trip or gift and whether it is part of a pattern.

Likewise, the committee manual outlines the crious criminal statutes and how severely the House should discipline members who violate them.

For instance, the manual defines the bribery law and points out the difficulty of proving "corrupt intent" by a politician who gets something of value in return for an official act.

The closely related gratuity law, which was used to convict former Sen. Daniel Brewster (D-Md.). doesn't require proof of "corrupt intent" and usually refers to gifts given as a reward for past services, rather than as inducement for future acts, the manual points out.

Violatings of the bribery statutes appear especially hard to prove in the Korean criminal investigation because of the difficulty of proving that some one voted for continued U. S. aid to Korea because of some gift or payment.

Brewster was convicted of having received several thousand dollars from a Spiegel, Inc. mail-order house lobbyist in return for his vote on postal legislation.

An earlier Brewster conviction was overturned when an appeals court ruled the jury wasn't told the difference between a gratuity and a legal campaign contribution.

Such fine points are sure to be argued by any member accused of taking money from the South Koreans. Already those who received cash payments from Tongsun Park and others have claimed they didn't know he was a foreign agent or that they thought his money was a legal donation!

Until 1975 it was legal to accept campaign contributions from foreign nationals. But the House committee manual points out that members should have wondered why Park, the flamboyant Washington party giver and rice salesman, was so generous with his envelopes full of $100 bills.

So far, less than a dozen past and present members have acknowledged taking contributions from Park or other South Koreans. Thus estimates of 115 members who have visited South Korea and been entertained there. It is uncertain that even the House committee's strict manual of offenses would hold members liable for attending dinners on official visits to other countries.