ONCE THE WORLD SERIES and political campaigns are over, television executives will have to begin dealing with a painful truth.This has been, thus far, a bad fall for programmers of network TV. Even as the political campaigns and debates have demonstrated the unique capacity of television to reach a mass audience, to serve as the electronic town hall that Ross Perot and others have discussed, the networks have witnessed nights where viewers are defecting from traditional series in perilous numbers. Four weeks into the 1992-93 term, the webs are already backpedaling rapidly away from some of their more aggressive programming gambles, no new series has emerged as a major hit, there are several nights when only a single network is enjoying acceptable shares and independent candidate Perot’s paid political ads are outdrawing most new series. NBC and CBS already have been forced to deploy movies on Friday and Saturday, respectively, after striking out with new shows, leaving only low-rated but critically acclaimed sophomore series “I’ll Fly Away” and “Brooklyn Bridge” in their wake. Other nights have suffered enormous declines in share on a four-network basis. The first four weeks of last season, for example, the three networks and Fox combined to average a 79 share from 8-9 p.m. Thursday. This year, they total a 70 share, and the four services have seen an equally disheartening 8 share loss in the same hour Saturday. AFTER THE OPTIMISM that attended the finish of the 1991-92 season, when the decline in network viewing ceased, the webs are again looking at a likely drop vs. the previous year, and Fox Broadcasting Co.–in many ways the biggest success story of last season–could face significant growing pains as it seeks to add new nights. The fall does have a few bright spots, but even the new series performing well in protected time periods (“Love and War,”"Hearts Afire” and “Hangin’ With Mr. Cooper”) are generally doing worse than the shows they replaced. Of the 33 new shows introduced this year only five have been canceled, but that’s more a tribute to diminished expectations than their success. Not a single new show placed behind an established series has shown the potential of becoming a stand-alone hit, as “Home Improvement” had by this time last season, and no new show in an unprotected time period has upset the balance of power in its slot. Even from a quality standpoint, few of the new shows have won the sort of acclaim showered on members of last year’s flock, and most of the best sophomore series–renewed this year for no other reason than critical acclaim–will be hard-pressed to live to see a third season based on their performance. LOOKING AT MATTERS network by network, “The Young Indiana Jones Chronicles” has failed to make any headway whatsoever for ABC on Monday, the web’s 10 p.m. Tuesday jinx continues (in more muted form) with “Going to Extremes,”"Civil Wars” has been all undressed with no place to go and its Sunday movie continues to be a distant third in that competition. That said, ABC is still the only network to have posted year-to-year gains at this point, up in both households and key demographics. For all the talk about targeting 18- to 49-year-olds vs. households, NBC and CBS have been equal-opportunity programmers, down by each criterion compared to last year. Outlining NBC’s woes would take the better part of the rest of this space, but in a nutshell its new Saturday comedies have proven a major disappointment, as is “Rhythm and Blues” and, for that matter, “A Different World.”"Unsolved Mysteries” has lost a little steam and “Seinfeld” proved no match for “Home Improvement.” At CBS, Monday appears slightly weaker with the new shows in the mix, Wednesday and Thursday are being held together by the news division (“48 Hours, “”Street Stories”) and Saturday is already DOA. Fox is springing its share of leaks as well, with “The Heights” and “Melrose Place” both proving there’s a limit to the public’s appetite for ensemble dramas and the early and late hours of the Sunday lineup still a drag on the night. The bright spots? “Home Improvement” has clearly worked on Wednesday at ABC, “Roseanne” has reasserted its monster-hit status, and “The Commish” continues to blossom into a second-year success. CBS’ Monday shows appear to be holding up fairly well, the Friday gambit is a qualified success, “60 Minutes” still spurs its domination of Sunday and the Tuesday and Sunday movie franchises remain extremely powerful. NBC’s Tuesday lineup clearly isn’t the disaster it might have been, ditto for Sunday from 7-9 p.m., “The Fresh Prince of Bel-Air” and “Blossom” have lived up to the latter’s name Monday and, in the good news-bad news department, there aren’t any established hits left to go elsewhere next season. At Fox, “Martin” has become the first Thursday series to maintain its lead-in from “The Simpsons,”"Cops” has grown into a solid hit with help from network ineptitude, “Beverly Hills, 90210″ took its teen audience to Wednesday and the 8 -10 p.m. Sunday block continues to perform admirably. The World Series will be over no later than Sunday. Election Day is less than two weeks away, and the Nielsen November sweeps begin Oct. 29 and end Nov. 25. That last date normally signals a minor lull until the new calendar year, a time to regroup and plan for January. Unless matters improve, however, and some of these first- and second-year series begin to catch on, the networks could be facing a very long winter.