John Zacarro, husband of the former vice-presidential candidate, Geraldine Ferraro, pleaded guilty yesterday to a charge of scheming to defraud in connection with the purchase and financing of five apartment buildings. As soon as his indictment by a Manhattan grand jury was disclosed, the New York real estate broker admitted that charges in the indictment involving falsified documents were true. Sentencing has been set for Feb. 20; Justice George Roberts said that in return for his guilty plea, Mr. Zaccaro will not be sent to prison. But although the crime is a misdemeanor, not a felony, this is serious business.

During the recent presidential campaign, a number of charges were made about the Zaccaro family and its financial affairs. Some, we thought, were so remote both in time and in relation to Rep. Ferraro's candidacy as to be irrelevant. What did it matter that her father-in-law, decades ago, had vouched for the character of a Mafia leader on a gun-permit application? Why bring up minor gambling charges leveled against her parents when she was 8 years old?

Other accusations reflected more directly on the candidate and her husband. Rep. Ferraro's failure to disclose financial information about her family on House ethics forms, for example, was a serious lapse. And Mr. Zaccaro's mismanagement of the trust funds of an elderly widow indicated at best an abysmal ignorance of his responsibilities as a court- appointed conservator. But neither of these situations involved criminal conduct, and both could be generously attributed to a failure to understand what the law required. Conviction on criminal charges, though, is a whole new matter, and one not so easily explained to the electorate.

After the election -- and even during the campaign -- most political observers believed that Geraldine Ferraro would recoup her loss by running for the Senate from New York in 1986. She may still do that, and time and the candidate's own ability and political savvy may diminish the impact of this week's court action involving her husband. But a new race will not be easy, either politically or in terms of its impact on the Zaccaro family. The candidate's future, which seemed so very bright six months ago, looks different now.