Since the redistricting of 1980, Republicans have been crying "foul," complaining that Democratic-controlled state legislatures have redrawn voting districts deliberately to keep the GOP from winning scores of congressional and local legislative seats across the country.

On Monday, the Republican National Committee will finally get its day in the U.S. Supreme Court on the issue of gerrymandering, in a case all sides agree is potentially the most significant in two decades.

But in a twist its lawyers deem "unfortunate," the RNC finds itself supporting the Democratic side of the suit before the court. The GOP committee has filed a friend-of-the-court brief on behalf of Indiana Democrats, who argue they were denied equal protection of the law by a plan drawn by their state's Republican-controlled legislature.

"Gerrymandering is a two-edged sword with which members of a political party may either carve or be carved," the RNC brief notes, searching for just the right metaphor to describe its ticklish position.

The odd bedfellowship has been the source of some amusement in political circles here, but the issue itself is no laughing matter.

"We think there are up to 40 safe Democratic congressional seats in the country that would become competitive if the court were to rule in our favor," William Greener III, the RNC's political director, says.

Democrats scoff at his estimate, but with their party controlling both legislative chambers in 27 states, and the Republicans both in just 11, it's no mystery why the Republicans want the courts to curb the excesses of partisan cartography.

At the top of the GOP grievance list stands the state of California, where an unbashedly partisan map drawn in 1981-82 by the late Rep. Philip Burton (D) has all but guaranteed Democratic hegemony in that state's congressional delegation until 1990.

Last year, Republican congressional candidates in California received more votes in the aggregrate than Democratic candidates, but they won only 17 of the 45 seats. The RNC says that, without changing a single vote, it is possible to draw a map that would have yielded 28 GOP winners from the same election.

California Republicans are suing over the Democratic plan, but the Indiana case reached the Supreme Court first. So Indiana, in effect, has turned into a proxy war for the larger stakes of California and beyond, with the sides devilishly reversed. The California Democratic Party not only has filed a brief in support of the Indiana Republicans, but it also has gone to the trouble of hiring a publicist to get out its side of the story.

The heart of the legal issue is this: Is a person's membership in a political party a constitutionally protected right? The national Republicans and Indiana Democrats argue that it is, and that, therefore, any plan drawn with the express purpose of diluting the voting strength of a party is unconstitutional.

Over the years, the court has been reluctant to step into the "political thicket" of redistricting. Until 1962, it was guided by Justice Felix Frankfurter's dictum that "it is hostile to a Democratic system to involve the judiciary in the politics of the people." Frankfurter wrote that admonition in 1946, in refusing to consider a case in which Illinois had an 800,000-person difference between its largest and smallest congressional districts.

By the 1960s, demographic shifts had made the overrepresentation of rural areas a widespread problem, and in a landmark 1962 ruling the Supreme Court decreed that districts must be equal in population. It has ruled since that redistricting plans are also unconstitutional if they dilute the voting strength of racial groups.

But, so far, the court has shied away from the issue of whether gerrymandering -- deliberately drawing voting districts to favor one political party -- is unconstitutional.

In the Indiana case, the state GOP hired a Republican consulting firm, Market Opinion Research of Detroit, to produce a computer-drawn map that would maximize GOP representation in the state legislature. The computer did its work well, and in 1982, Democrats found themselves winning 51.9 percent of the votes cast for statehouse, but only 43 percent of the seats.

Last December, a U.S. appeals panel ruled 2 to 1 that the Indiana Democrats had been denied equal protection and ordered the state to devise a new map by 1986. The Supreme Court is hearing an appeal of that decision.

"My guess is that the Supreme Court will . . . look at these computer maps and say, 'If we don't do something now, it's going to be Katie bar the door in 1990,' " RNC attorney E. Mark Braden said.

The RNC has picked up support from the American Civil Liberties Union and from Common Cause, the self-styled citizens lobby. But other nonpartisans argue that the courts should leave politics to the politicians. They say the danger of gerrymandering is overblown, and that it is far more common for incumbents of both parties to work out plans that protect each other.

"I'm dismayed at the courts getting into this whole area," Norman Ornstein, a congressional expert at the American Enterprise Institute, said. "I find it especially ironic that conservatives are trying to get the most activist kind of judicial remedy for their grievance."

Ornstein argues that a disparity between the aggregate vote and seats won is not proof of gerrymandering. He notes that the gap in 1984 -- when the GOP won 47 percent of all congressional votes cast and 42 percent of the seats -- is lower than the average gap of the past two decades.

Majority parties, he said, get a built-in "bonus" from the winner-take-all system of elections in this country. (In the most extreme theoretical case, if the majority party won all of its races with 51 percent of the vote, it would get 100 percent of the seats).

Braden counters that gerrymandering not only favors Democrats, but also, by protecting incumbents of both parties, creates an "ossified" Congress that does not change with a changing electorate. He said that if the RNC wins the case, it will consider suing in perhaps eight other states.