THE DECISION came in federal court in New York and concerned a law that applies solely to New York City, but it sends reverberations through the South and provides at least one answer to a question raised by Jesse Jackson in the 1984 presidential campaign. The law requires runoffs in New York's mayoral primaries when no candidate gets as much as 40 percent of the vote; it was passed after the 1969 election, in which Mario Proccacino won the Democratic nomination with 33 percent of the vote and then proceeded to lose the general election.

In August a federal district judge ruled the law unconstitutional, saying it was aimed at least partly "at preventing minority votes from gaining political power"; the Second Circuit appeals court reversed that decision the other day. Its decision makes sense. New York's law was biased not against racial minorities, but against any splinter candidates. It is not unconstitutional for a state to require that a candidate should win a majority, or some lesser percentage, in order to be elected, just as the Constitution requires a president to win a majority in the Electoral College.

The New York decision does not automatically uphold the southern state runoff requirements attacked last year by Jesse Jackson. Some of these may have been enacted originally with an aim of reducing the chances of candidates supported by blacks. However, their effect today is quite different because electorates, North and South, are increasingly willing to look beyond the race of the candidates. California, 7 percent black, voted 48 percent for a black candidate for governor in the last election. Such white majority cities as Los Angeles, Chicago, Philadelphia and Charlotte, N.C., have black mayors, just as black-majority Baltimore has a white mayor. These officials have proven, by a test more stringent than any the courts can apply, that they represent the interests of their constituents of all races.

The argument against runoff elections is posed most poignantly if you imagine a white-majority electorate obdurately unwilling to give any of its votes for a black-backed candidate. Then, you might say, letting the latter win with less than a majority in a multi- candidate race is only fair. A result fair to the minority? But what if the whites are not only obdurate but shrewd enough to unite on a single candidate? Would you urge a court to declare the second-place finisher the winner because the voters supporting his opponent were motivated by racial prejudice?

Happily, there are fewer and fewer political situations in America resembling this hypothetical one, and more and more where candidates are judged on their merits, not their race. The legal attack on runoffs is undermined by the decision in New York; politically it has always been a diversion from the hard but rewarding work of building a more tolerant and fair-minded politics.