The off-year elections of 1985 are weeks away. The fields are still forming for the mid-term campaign of 1986. Only political junkies are monitoring maneuvering for the 1988 presidential campaign. But both parties are engaged in the political battle of 1991 -- the struggle for control of congressional redistricting in the 50 state legislatures.

It's a two-front war, legal and political; and the stakes could be as big as control of the U.S. House in the 1990s. The lawyers had their day in the Supreme Court last Monday, when the justices heard arguments on an Indiana case in which they are being asked to to decide whether gerrymandering of district lines violates individuals' constitutional rights.

Without waiting for the outcome of that case, Democrats and Republicans are gearing up for the battle. The GOP actually declared war more than two years ago, when Republican National Committee Chairman Frank J. Fahrenkopf Jr. announced what is now called "the 1991 plan," a targeted, multi-faceted program aimed at "dramatically increas ing the number of Republican legislative seats and governorships in preparation for the crucial 1991 reapportionment."

The GOP campaign claimed initial success in gaining more than 300 legislative seats and one governorship in the Reagan landslide of 1984. It will receive an important test next month in New Jersey, where the GOP hopes to capitalize on the expected reelection victory of Gov. Thomas H. Kean (R) to overturn the eight-seat Democratic majority in the state assembly and the six-seat Democratic edge in the state senate.

Last week, worried Democrats decided that they had better try to get into the contest before the 1986 elections. Their vehicle is a newly charted enterprise called Project 500, a consortium of party committees, unions and independent political groups, headed by Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee.

According to Paul Jensen, a Labor Department official in the Carter administration and the union liaison in Walter F. Mondale's presidential campaign, who will direct the effort, its goal is to "elect or retain 500 legislators in marginal districts and states in the next five years." Thus, the name, Project 500.

Jensen's operation is drawing on the resources of Pamela C. Harriman's Democrats for the '80s PAC (political action committee), the liberal National Committee for an Effective Congress, the American Federation of State, County and Municipal Employees (AFSCME), as well as the Democratic National Committee, Senatorial Campaign Commitee, and the associations of governors and state legislative leaders.

Leaders of all those groups are on the executive committee along with Coelho and veteran party fund-raiser Nathan Landow of Bethesda.

Coelho, the prime mover in the project, said Friday that "the Republicans are smart" to have started early, "but we can win as long as we aren't caught napping."

The prize over which the parties are starting this long-range campaign is control of the congressional redistricting decisions that will follow the 1990 Census. Early projections are that 20 states will gain or lose House seats. The most dramatic projected shifts are gains of four seats each for Florida and Texas, three seats for California and two for Arizona. Losses of five seats are projected for New York, three seats for Pennsylvania and two each for Illinois and Michigan. Internal population shifts will require lines to be redrawn in all states except the handful with single-member, at-large districts.

In 1981, when the census figures showed a similar population shift from the industrial "Frost Belt" states to the leisure-retiree-services-high tech "Sun Belt" states, Republicans boasted that they would be the beneficiaries. They advertised that they were ready to enter the districting battle with the most sophisticated computers and survey data.

What they forgot was that they needed votes and that in such burgeoning states as Florida, Texas and California, Democrats were solidly in control of the legislatures that would redraw district lines. The result was that in 1982, Democrats won 10 of the 17 newly created House seats and made Republicans swallow many of the lost districts.

Fahrenkopf vowed that it would not happen again and has been candid to the point of controversy in telling party contributors and supporters there is no hope of achieving a Republican majority in the House unless the GOP first overcomes its severe disadvantages among state legislators and governors.

Buttressing Fahrenkopf's argument is the fact that in 1984, Republicans won 47 percent of the popular vote for Congress but only 42 percent of the seats, and that parity of seats and votes has not been obtained in any congressional election since 1952. Although such scholars of congressional elections as political scientist Norman J. Ornstein of the American Enterprise Institute argue that "gerrymandering is at most a very minor factor in Republican underrepresentation in Congress," Republicans are going after the legislatures as if they were the key to the future.

They are spurred by such examples as California, where a classic Democratic line-drawing exercise corralled Republican strength so efficiently that Republicans won a plurality of the vote in 1984 but only 18 of 45 seats.

Republicans control 16 of the 50 governorships and 11 of the 50 legislatures, with control of 10 other legislatures split between the parties. But the margins of control in many states are narrow enough to encourage hopes in both parties for switches.

California will obviously be targeted by both Project 500 and the 1991 Plan because of the high stakes. Democrats have a 25-to-15 edge in the state senate and a 47-to-33 lead in the assembly. In Texas and Florida, two other states that will gain many House seats, the disparity in the legislative strength is greater, but the GOP is mounting major campaigns for the governorship in 1990.

The Republican operation is tied closely to the basic party-building program of Fahrenkopf's committee, which focuses on systematically upgrading the staff, facilities, finances and operations of 750 targeted county GOP organizations.

Terry Wade, the RNC communications director, said special efforts are directed by the committee's field staff to recruit and train legislative candidates and campaign managers. "It's basically a grass-roots, state-level campaign," he said.

Coelho has estimated that the opposition is pumping $15 million into the 1991 Plan, but Wade said the close links to other organization efforts make it impossible to put a price tag on the effort. A wrinkle of the campaign-finance law -- already well-exploited by the GOP and eyed with envy by the Democrats -- allows direct corporate and union contributions to be used to aid legislative candidates in most states.

Parallel to but separate from Fahrenkopf's effort, former Delaware governor Pierre S. (Pete) duPont, IV, has sponsored a group called GOPAC since 1979, which has raised money nationally to support nonincumbent Republican legislative candidates. Carter Hendren, who heads GOPAC's staff of 14, said the organization raised $2.64 million in the 1983-1984 election cycle and hopes to almost double that in 1985-1986. GOPAC is seen by many people as serving the corollary purpose of furthering duPont's 1988 presidential ambitions.

In addition, a number of conservative groups, such as the American Legislative Exchange Council, the Free Congress PAC, the Fund for a Conservative Majority and the Public Service Research Council pool information on recruiting legislative candidates and individually funnel support to their campaigns.

Compared to these efforts, the just-starting Democratic drive will be modest in its goals. Jensen said the aim is to raise about $2 million for the 1985-1986 cycle, for direct contributions to legislative candidates, training, recruitment and support services.

He said the staff would be small, with targeting assistance coming from NCEC, research from the Harriman PAC and field work from AFSCME and other unions that are expected to join the effort.

While two years behind the Republicans, the Democrats are five years ahead of their last redistricting effort when virtually nothing was done from the national level until the legislatures went into session to start drawing maps in 1981.

Like baseball teams that depend on their farm systems, both parties have learned that long-range planning is essential for survival.

Political researcher Maralee Schwartz contributed to this article.