FOR ALL OF ITS 30-odd years, the Voice of America has been tugged by two contrary missions: to explain American foreign policy and to be an objective and comprehensive source of news. The first mission requires this official American radio station to hew to the official line, the second to be at times irrelevant to policy or even a hindrance to it. This is the tension underlying the State Department's move to prevent a VOA correspondent from meeting with the Polisario guerrillas in the Sahara. The United States does not support their cause. To have an American civil servant in contact with them was something American diplomacy could not abide.

To some at VOA, this latest instance of State Department intervention is an unacceptable blow to the station's credibility. They see the tension between VOA's different missions not as built-in but as gratuitous, something to be removed by making VOA as independent of government control as a commercial news operation. To them, the committee that VOA and its parent organization, the United States Information Agency, is setting up to study the situation of VOA's 16 foreign correspondents is a dodge.

We repectfully dissent. One of the strengths of the Voice is the Zeal with which its staff pursues journalistic professionalism. Fortunately, there are other ways by which the Voice can keep up its credibility, notwithstanding the relatively few circumstances in which policy considerations cramp its style. Specifically it can ensure that what its millions of listeners do hear is accurate and fair.

The guest for a code of internal regulations and an organizational structure in which VOA can both report the news and support official policy cannot fail to vex conscientious bureaucrats. It is a quest made harder by the fact that the Voice, unlike, say, the British Corporation, speaks in the name of a superpower - a country to which a substantial capacity to influence events is commonly ascribed. Some part of the BBC's considerable reputation flows, we suspect, from its listeners' awareness that Britain no longer has the global influences that it wielded in the days of empire. This is to say that "credibility" in international broadcasting can't be separated from the nature and size of the country broadcasting. We are confident, nonetheless, that the VOA can do, or continue to do, its multiple jobs.