Is Hatfield issue one man's mistake, or institutional flaw?

Each year as a few members of the US Congress step into legal quicksand, they raise anew public doubts about the integrity of the nation's legislature. But the latest case is perhaps the most jarring because it touches the man who for three terms in the US Senate has epitomized high principles and a deep religious conviction that was evident but never touted.

Sen. Mark O. Hatfield (R) of Oregon admitted this week that he committed ''an error in judgment'' when his wife, a realtor, accepted $55,000 in fees from a Greek financier whose trans-Africa oil pipeline Senator Hatfield had supported. He and his wife did nothing wrong except failing to avoid the ''appearance of impropriety,'' he said.

The legal issues now rest with the Senate ethics committee and the Federal Bureau of Investigation. But the fall from the high moral pinnacle has already occurred, and questions linger about whether this is a story of one person who made a mistake or of a flaw in the institution.

William M. Broadhead, a Detroit lawyer who served three terms as a Democrat in the House of Representatives and was once considered to have a shot at becoming governor of Michigan, dropped out of politics in 1982 largely because of what he saw as the lopsided influence of money on Capitol Hill.

''I think the members of Congress are overworked and underpaid, and I think that's where the problems arise,'' says Mr. Broadhead, during a brief visit to Washington. ''This is a very rich time and a very rich city.''

He defends his former colleagues generally, calling them ''highly motivated, ethical'' officials, but he adds, ''I think the temptations are greater these days.'' Broadhead estimates that most of the members of Congress have expertise worth salaries of more than $100,000 in the private sector. ''Many of them know that,'' he says. ''That's one of the reasons why they get led into temptation.''

The temptations are multiplied, says Broadhead, because lawmakers are ''surrounded by lobbyists whispering in their ear'' and ''tend to associate'' with people in high-income brackets.

''I felt that one of the problems I had was who would come to see me,'' he recalls of his congressional experience. ''It was overwhelmingly people who were fairly well off. Who comes to Washington? The more wealthy and affluent of your constituents.''

Nonetheless, the former representative argues that most members of Congress are ''very, very highly ethical and principled people'' whose standards are higher than their counterparts in private business.

Thomas E. Mann, a close congressional observer and executive director of the American Political Science Association, says that despite recent charges against individuals, ''I don't think the Congress is crooked. I think it's cleaner than ever.''

Dr. Mann bases that belief on anecdotal history ''about the norms guiding people in earlier periods when trading for legislative favors was rather common'' and the requirement that officials now disclose their private finances.

''We expect more of public officials, and by and large we get it,'' Mann says , adding, ''many are tempted and some give in to temptation.'' The chief concern , he says, is when a lawmaker uses his public office for private gain, which is the pivotal question in the Hatfield case.

The Oregon senator indicated this week that his problems stem chiefly from having a spouse who has a separate career.He tied his case with that of Rep. Geraldine A. Ferraro, the Democratic vice-presidential nominee, who is under attack because of failure to disclose the dealings of her husband, a wealthy real estate developer.

Mann draws a sharp distinction between the two cases, however. At issue in the Ferraro dispute is whether she improperly exempted herself from reporting her husband's business dealings. She is also being asked to explain family loans to her 1978 campaign that were later found to be illegal.

''This is not corruption in the sense of improper use of office for financial gain,'' Mann says, calling the Ferraro and Hatfield cases ''of a completely different order.''

The fact that such issues come to the fore are proof of ''what standards we hold (members of Congress) to,'' Mann says. ''We hold them to high standards, higher than ever before.''

''I honestly do not believe it's widespread,'' he says of officials using their offices for financial gain. ''Vigilance keeps it at a minimum,'' he says, citing public exposure given to financial disclosures. But he also concedes that with each allegation, the public concludes that dishonesty is prevalent among many congressional representatives - ''except their own.''