THERE WASN'T MUCH DEBATE before the Senate decided by voice vote recently to authorize a new special prosecutor to pursue the investigation of alleged Korean influence-buying on Capitol Hill. One can think of several reasons why the idea was accepted so readily. For one thing, it never hurts for senators to endorse a vigorous probe of a scandal that so far seems to be centered in the House. Beyond that, the plan advanced by Sen. Don Riegle (D-Mich.) would give the Attorney General 90 days either to find the allegations "unsubstantiated" or turn the matter over to a court-appointed prosecutor. These options should appeal both to those who want a full inquiry and to those who prefer to have the Justice Department contain the case and wrap it up soon.

Congressional Republican leaders have been especially skeptical of a Democratic administration's ability to investigate affairs involving prominent Hill Democrats. But here, too, some other factors may be in play. On June 9, for instance, House Minority Leader John J. Rhodes (D-Ariz.) called for a special prosecutor partly as a way of stopping Justice Department leaks. Oddly enough, two days earlier The New York Times had carried a leaked report that the scandals may touch some prominent Republicans.

We don't want to be unduly cynical about this. It is striking, though, that no advocate of a speical inquiry has set forth any specific evidence that the Justice Department is incapable of handling this case. The sheer length of the investigation does not prove that Justice has been dallying. The case is very subtle and complex, some key figures are out of the country or shielded by diplomatic immunity, and bribery and influence-peddling can be devilishly hard to prove.

To some, those very factors suggest that at least the appearance of political conflicts is unavoidable, so that a special prosecutor should be named to bolster public confidence. But that line of argument assumes that regular agencies of justice cannot be trusted to handle politically sensitive cases at all - and that special prosecutors are inherently more upright and effective and untainted by politics or personal ambition that the people entrusted with the day-to-day enforcement of the nation's laws. If that view is to be one of the enduring legacies of Watergate, it promises to be a very ingenuous and dangerous one.

It is wiser, we think, to encourage - indeed, to demand - strong, nonpartisan investigations by the Justice Department and to resort to a special prosecutor only in rare cases where the political complications are acute. Such cases might involve serious charges against the President, allegations of corruption high up in the Justice Department itself, or situations in which the Attorney General himself concludes that a political conflict exists. Significantly, the Senate approved a standby mechanism along these general lines in the very bill to which the Riegle amendment was attached.

At some point, the Korean affair might become the sort of case in which a special prosecutor is required. Right now, however, the Justice Department should press ahead. If members of Congress want to prove their commitment to rooting out corruption on the Hill, they can best do so by investigating the scandal vigorously themselves and dealing firmly with colleagues whose conduct, while perhaps not provably criminal, has abused the public trust.