THE SUPREME COURT changed the ground rules of national politics a few days ago in upholding, 6 to 3, "a new affirmative right of access to the broadcast media" for candidates in federal campaigns. What this will do to future media campaigns will not become clear until candidates for the 1982 House and Senate races have been certified and begin planning campaign strategy. For the moment, however, the court majority has eroded badly -- though not entirely destroyed -- the ability of the networks to limit severely the number of hours devoted to paid political programming. Whether or not a particular candidate for federal office will benefit from early (or persistent) exposure on the tube will depend in the future mainly upon the size of his campaign war chest and the judgment of political managers. But it will not depend on the tolerance of commericial television executives concerned mainly with the effect on audience ratings of replacing scheduled shows with one-shot political ads. In that sense, the decision is healthy and reinforces public scrutiny of adversaries and their arguments within American broadcasting.

Both Chief Justice Burger's majority opinion and the dissent written by Justice White recognized squarely the conflict of First Amendment rights and interests inherent in the case. Mr. Burger and his colleagues held that, in this instance, the rights of candidates -- and of those watching or listening to them -- took precedence over the editorial (or business) judgments of broadcasters. Who should decide whether or not requests for paid political air time unnecessarily disrupt normal programming? According to the majority, current statute law gives the Federal Communications Commission sufficient regulatory authority to override the networks when the commission finds a candidate has been denied the right of "reasonable access" for such ads.

Although the court described that right as limited and applicable "only to legally qualified federal candidates . . . once a campaign has commenced," serious questions remain about how the new standard will be enforced. For one thing, every campaign season "commences" earlier than the last one. In the case under review, President Carter's re-election campaign tried as early as October 1979 to purchase 30 minutes to prime time on all three networks to be scheduled in early December. The FCC found that not only ABC'c and NBC's flat turndowns, but also CBS's counterproposal for two 5-minutes time slots constituted an unreasonable denial by the networks.

Armed with the Supreme Court's mandate, the FCC presumably will strive, like a Solomon of the electronic kingdom, to divide television's prime-time baby between the possessive mothers of politics and network pap. An example: Candidate X asks to preempt "Dallas." When Candidate Y, the opponent, requests the same slot on another week, CBS offers instead "The Dukes of Hazzard" or (gulp) "The Incredible Hulk." Has CBS honored -- or violated -- the "reasonable access" provision by its actions? For that matter, can the networks be expected to accommodate all candidates in a primary or general election, however minor their party, who plunk down the necessary funds to purchase a large block of time? And will the FCC use its new authority dispassionately? The commission's initial 4-3 straight party-line vote, Democrats vs. Republicans, on the Carter-time question does not hold out much encouragement for the future in that respect.

If the networks hang in there, of course, the expanding cable tevevision systems and even newer twoway video technologies may transform the lockhold the three commercial networks now have on primary contact between politicians and the electorate, thereby allowing events to overtake last week's Supreme Court ruling. Somehow we doubt that the television elite funds much consolation in that thought, nor even in the sensible point made by Justice Stevens in a separate dissent that "anyone who listened to the campaign rhetroic that was broadcast during 1980 must wonder how an impartial administrator could conclude that any presidential candidate was denied 'reasonable access' to the electronic media."