THE decided tilt California has taken toward the GOP in a new redistricting plan has buoyed Republicans from Sacramento to Washington, D.C.It gives Republicans a chance of controlling one house of the state Legislature for the first time in 20 years and the possibility of wresting control of the nation's largest congressional delegation. The gains are only a will-o'-the-wisp at this point, however, because the proposed map released recently by a court-appointed panel may yet be challenged, and Republicans still have to field appealing candidates and get them elected. "There is a big difference between redistricting and the reality of winning elections," says Bruce Cain, a political scientist at the University of California at Berkeley. "All we can say at this point is that the Republicans have laid the groundwork for a big shift." California is the crown jewel in the battle over redistricting. Its congressional delegation, now 26 to 19 Democratic, will represent 12 percent of the United States House of Representatives next year, when seven new seats are added because of populations shifts. If Republicans were able to pick up eight to 10 seats - a distant but not impossible goal under the proposed plans - it would give a major boost to the GOP drive to strengthen its base in Congress in the 1990s. "I think a split in the California delegation is conceivable," asserts Benjamin Ginsberg, chief counsel to the Republican National Committee. Republicans have been doing well by the pens of judges lately. A recent court-designated redistricting plan in Illinois, which will lose two congressional seats in 1992, favors the GOP. The losers are most likely to be white ethnic Democrats. Recent elections that put Republicans in the majority in the New Jersey Legislature will help GOP redistricting chances there. The party harbors hopes in South Carolina and Michigan as well. Conversely, a plan in Texas that would benefit the Democrats has passed its first hurdle - screening by the US Department of Justice - though Republicans and Hispanics are fighting it in court. Democrats are also heartened by early maneuverings in North Carolina and Georgia. One Democratic analyst concedes his party may lose 10 to 12 seats as a result of redistricting, but he disputes the idea there will be any substantial change in the 268 to 166 Democratic majority in Congress. "Overall, I would say the redistricting process has been a wash so far," says Doug Chapin, a researcher at Election Data Services, a political-consulting firm. The new California plan was drafted by three retired judges appointed by the state Supreme Court to break an impasse between the Democratic-controlled Legislature and Republican Gov. Pete Wilson. Hearings on the proposed map are scheduled for Dec. 13. It is expected to be adopted by the high court in January unless the Legislature and governor agree on an alternate plan or Democrats are able to draft one that would survive a Wilson veto. Both are unlikely. Although the new map hurts incumbents of both parties, it is beneficial enough to the Republicans that they aren't likely to bargain with the Democratic leadership for anything different. The plan, however, could face opposition in the courts from Latinos who, even though they would make major gains, are upset that several other Hispanic seats weren't created. Disgruntled Democrats may have grounds for appeal, too, since the population of congressional districts varies by more than some experts believe is allowed under federal law. The map carves up the districts of some of the state's most influential Democrats, including Reps. Henry Waxman, Howard Berman, and Vic Fazio, and state Senate President Pro Tem David Roberti. All would likely face tougher reelection bids. Several prominent Republicans have seen their seats splintered as well, though Democratic incumbents take a far larger hit. By several estimates, the number of reasonably safe Democratic and GOP congressional seats under the plan would be about 24 each. The remaining four would be a toss-up. In the state Legislature, experts say Republicans would have the best chance of regaining control of the Assembly, now 47 to 33 Democratic. The 24-to-13 Democratic advantage in the Senate could narrow as well. The GOP has been a minority in both houses for all but two of the last 30 years. Though Governor Wilson has been lauded for successfully forcing the redistricting issue into the courts and getting a favorable plan, he still has to lead his party through the election process. That means candidates, money, and messages. The conservative and moderate wings of the party have been sharply divided over taxes, gay rights, and other issues. President Bush's popularity will also be a factor. "This is a good competitive plan. It isn't a Republican plan," says Sal Russo, a GOP strategist. "Good candidates are going to win, whether Democrat or Republican." Democrats think they have the themes right now that best resonate with voters. They also are likely to field more incumbents. Thus, one political analyst cautions: "Let's not assume that because lines on a map favor the Republicans it is a slam dunk for them."