IT HASN'T exactly gotten headlines east of the Sierras, and it's not even well known yet in California. But Proposition 39, a measure to change the way California Assembly and Senate district lines are drawn has the potential to dramatically affect the balance of national political power.

How can this be? The current district lines were drawn by Democratic majorities in the legislature, signed by Democratic Gov. Jerry Brown, and approved by a state supreme court, six of whose seven members were appointed by either Mr. Brown or his father. They provide safe seats for Democratic incumbents and hence tend to guarantee Democratic majorities for a decade. The congressional district lines, drawn by the late Rep. Phillip Burton (D-Calif.), have given enough Democratic incumbents a safe seat to produce a delegation of 28 Democrats and 19 Republicans in President Reagan's home state. Some of those Democrats -- Tony Coelho, Leon Panetta, Henry Waxman -- are among their party's leaders on Capitol Hill. Without the Californians, the Democrats would not have the controlling position they have in the House today and will have next year, unless the Republicans pick up more seats than they are now expected to win.

Proposition 39 threatens all this by taking redistricting away from the legislature and giving it to a panel of retired judges. Gov. George Deukmejian, a Reagan Republican who has been fighting the Democrats in Sacramento since he was elected two years ago, is backing Prop 39 strongly; he argues that it provides a fair, nonpolitical way to draw district lines. The Democrats entrenched in Sacramento and Washington are fighting it fiercely; they say it would bring hitherto nonpartisan judges into politics, and they chortle that the average age of retired judges willing to serve on such a panel is 82. Both groups of politicians are raising huge sums of money. The Republicans and their allies are using it to emblazon the state with the slogan "Fairness, Not Politics." The Democrats and their allies are running ads showing a politician tapping a judge on the shoulder and predicting that Prop 39 will drag judges into politics and make California "just like Chicago and other places with political machines."

Both sides have a point: there's probably no way to draw district lines that is above legitimate criticism. Even so, a lot is at stake. The requirement that each district have the same population imposes a limit on what politically minded redistricters can do, but the California redistricting in effect now plays a critical part in maintaining the Democrats' control of the legislature in the nation's largest state and in the U.S. House of Representatives.