Prosecutors make decisions every day to drop some cases and proceed with others. Most of the time these choices go unnoticed by the public, and it is rare that the judgments are second-guessed. But when the subject of an investigation is the international president of one of the largest, most powerful unions in the country who just happens to have strong political ties to the administration in power, an explanation is required. In the case of Jackie Presser, it had better be a good one.

For 32 months, federal investigators have been looking into charges that Mr. Presser approved payments of more than $250,000 in union funds to "ghost employees" who did no work. A Justice Department organized crime strike force in Cleveland recommended prosecution months ago, and the foreman of the federal grand jury in Cleveland publicly complained about Washington's delay in moving on the case. But now we are told that the government will not seek an indictment of the Teamster leader and has, in fact, already closed down the investigation.

It is hardly surprising that many people, considering the close political connections between the Reagan administration and the Teamsters, suspect the worst. The union was the only major labor organization to endorse the president in 1980 and 1984. Mr. Presser served on the 1980 transition team and has been received as a guest by the president. But the White House has been adamant in denying any involvement in the case, and Attorney General Edwin Meese, who had dealt with Mr. Presser on political matters, removed himself from the case at the time he went to the Justice Department last February.

Even if the Presser prosecution was not set aside for political reasons, there seems to have been an egregious foul-up somewhere. We are led to believe that Mr. Presser was some kind of an informant for the FBI and that indicting him would jeopardize some other -- more important? -- cases. But who authorized this arrangement and why didn't the strike force know about it? What is the relationship between the FBI and federal prosecutors in dealing with informers? And what are the ground rules, not only for this case, but generally, that govern decisions to drop some prosecutions in order to preserve others -- especially when public figures are involved?

The Senate Permanent Subcommittee on Investigations has already begun an inquiry into the government's handling of the Presser case. To put it mildly, the public deserves an explanation.