PENNSYLVANIA GOVERNOR Milton J. Shapp has done the right thing by promptly agreeing to repay $299,066 in federal subsidies that were obtained illegally by his short-lived 1976 presidential campaign. The governor says that he knew nothing about the bogus contributions and faked reports which, according to the Federal Election Commission, involved more than 40 of his fund-raisers and contributors. Even so, Mr. Shapp has decided not to exercise his right to contest the FEC's decision. "I believe it is right that the full amount should be paid back to the United States treasury," he said Friday.

Though the Shapp case is far from closed, it has already underscored three points about enforcement of the public-financing law. The first has to do with accountability; too often candidates confronted with evidence of shady fund-raising have washed their hands of it by claiming, however improbably, that they had no idea what was going on. As Mr. Shapp has learned the hard way, any candidate who gets public subsidies is financially liable in any case. If his supporters launder money, whether or not he knows about it, he's the one who may get taken to the cleaners by the FEC. That ought to be a powerful incentive for future candidates to keep much closer watch over their campaign financing.

The second point has to do with detailed disclosure; because donors' occupations and business affiliations had to be reported, the FEC's auditors were able to spot substantial contributions from people in modest jobs and clumps of gifts from employees in a few companies. Investigation showed that many of those donations had been faked. If only home addresses had been required, as in the past, the alleged frauds might not have been found out.

The probe of the Shapp case has so far engaged 10 FEC employees for about six months. That's major commitment to a relatively minor campaign, considering that the agency has only about 65 lawyers, auditors and investigators - and is simultaneously trying to review all the other presidential campaigns and last year's Senate and House races as well. Speed has already been sacrificed. A more serious casualty is thoroughness; FEC officials freely acknowledge that some reports are getting just a cursory review.

And that's the final point: The FEC has more business than its present staff and organization can handle very well. There is an obvious message here for the supporters of universal voter registration and public financing of congressional campaigns. Aside from their other problems, those proposals, especially the congressional campaign subsidies, would put very heavy new burdens on the FEC. One thing Congress should think hard about is how much manpower, reorganization and lead time the FEC would really need before there is even a chance that those programs could be soundly administered and policed.