TEAMSTER PRESIDENT Jackie Presser continues to be an irritant to the Reagan administration. The labor leader, whose union was the only one of any size to endorse President Reagan in 1980 and 1984, has been charged with corruption and racketeering. But the Justice Department's efforts to indict him last year were stymied after it was learned that he had long ago been promised immunity because he served as in informant to federal law enforcement authorities. The other day the president's crime commission, in an interim report, raised questions about the still unresolved Presser case and scolded the administration for "the appearance of impropriety" that had been created.

The incumbents are not the first or the only officeholders to court union endorsement, even whunions were under the control or influence of organized crime figures. The report cites, for example, the case of Anthony Scotto, described as a former Longshoreman's local and national official and a "capo" in the Gambino crime family. When Mr. Scotto was on trial for racketeering offenses, his character witnesses included the then-governor of New York, Hugh Carey, and three former mayors of New York City, John Lindsay, Abraham Beame and Robert Wagner. John Serpico, a Laborers Union leader with mob ties, according to the commission, had political friends "including some of Illinois' most prominent elected officials who took turns appointing him to the Chicago Regional Port Authority."

For politicians, union endorsements mean contributions and, more important, manpower and resources for their campaigns. In return, the unions hope to elect friends who will have labor's interests at heart. When the union is dominated by racketeers, though, the public has a right to know whether gangsters have bought anything else with their contributions and endorsements.

The commission did not charge -- and no evidence has been produced to show -- that this administration has been corrupted by its ties to Mr. Presser. It is this administration, in fact, that prosecuted his predecessor, Roy Williams; that first put a Teamster local into receivership, and that originally sought to prosecute Mr. Presser himself. But it is the withdrawal of that indictment -- presumably because of the earlier secret grant of immunity -- that has never been officially explained by the Justice Department. It creates, as the commission points out, the appearance of wrongdoing that leads to an erosion of public confidence in the government's commitment to fight organized crime. The Presser case will hound the administration until a full explanation of this episode is made.