This time justice was won by Republicans who challenged a Democratic state legislature's congressional districting plan. But both of the parties and indeed the whole American political system should benefit from the Supreme Court's strong new affirmation of the principle of one person, one vote.
This week's decision in a New Jersey case may open the door to litigation in other states, as the four dissenting justices argued. But litigation is a small price to pay for maintaining the constitutional standards that preserve democracy. And, when it comes to redistricting, states can avoid litigation by shaping their districts according to law in the first place.
Why did New Jersey's effort to redistrict on the basis of 1980 census figures fall short? Because, according to the high court, the legislators could have drawn districts of more equal population - and could not justify their deviation on acceptable grounds. Some variance might be justified by ''consistently applied legislative policies'' such as respecting city boundaries, making districts compact, avoiding contests between incum-bents.
In the absence of such justification, the Supreme Court made news by upholding a lower court's ruling against unusually small mathematical disparities in district apportionment. In the past it had never found a dis-crepancy of less than 1 percent in population equality to violate the constitutional demand for equal representation. Now it found 0.6984 percent to be too much.
The message seems clear. States must do their best to give every vote for Congress equal weight - or have a compelling reason for even the tiniest variation. It is the kind of message that the world - with so many countries still excluding whole categories of people from the ballot box - should hear from the highest tribunal in the land of the free.
Even with the first ''one-man, one-vote'' decisions of two decades ago, the Supreme Court went far toward eliminating what had been the worst form of gerrymandering - the skewing of voting districts by population to give unfair advantages to parties or candidates. The Voting Rights Act helped to check gerrymandering designed to discriminate against minorities.
The line of democratic progress is long and true. But its continuation depends on more than the tough mathematical demands in the New Jersey decision. Politicians in power still can jigsaw congressional districts into shapes that favor them while making the numbers add up right. Gerrymandering again.
Some states have tried to eliminate it by turning redistricting over to citizens' commissions. Citizens in any state can be alert to redistricting maps and work for gubernatorial vetoes - and, yes, litigation if necessary - to correct abuses.
Finally, as in all of the workings of democracy, the roots of reform lie in the regeneration of attitudes as much as in any reapportionment of votes.