COMMON CAUSE HAS STARTED a new campaign whose results won't be evident until the 1980s. In fact, 1980 is what this effort is all about. That's when the states will be reapportioning their legislatures and congressional districts in accord with the next census. Common Cause wants to change the way most states go about redrawing their political maps.

The group's prime target is gerrymandering - the time-honored technique of drawing a district into a peculiar shape in order to protect a party or incumbent or to weaken the influence of some voting bloc. The Supreme Court's "one man, one vote" decisions of the 1960s, far from ending this practice, have caused legislatures of be even more ingenious in shaping districts that are roughly equal in population but not politically competitive.

Common Cause argues rightly that competition is not likely to increase as long as legislators, with all their personal and partisan interests, design the maps. The citizens' group has therefore proposed a model system, based on methods used in Colorado, Hawaii and Montana, which would vest reapportionment power in nonpartisan, independent state commissions. The plan also includes strict antigerrymandering standards and prompt judicial review.

The group's proposal may not be the best approach everywhere, but it is certainly worth considering - especially in the 37 states, including Virginia, whose legislatures now redraw the lines themselves. Maryland has a somewhat different system, with the governor initiating a plan subject to legislative override, but that method, too, deserves review.

The citizens' group is right to raise the subject now. In virtually every state, changing the reapportionment rules will require constitutional amendment, and that tedious process can't be done by 1980 unless it is started soon. Moreover, the nation's politicans are already thinking hard about reapportionment. Many Republicans for example, fear that unless they can gain control of more state legislatures in a hurry, new districts drawn by Democrats may keep them out of state and congressional power for another decade. The GOP is gearing up for a real effort in state legislative elections next year. The party might also consider joining forces with Common Cause. For those who are out of power, fair reapportionment can be good politics as well as a very good principle.