The moment of truth is fast approaching for TV shows "on the bubble" — industry parlance for series facing cancellation because of wobbly ratings. Among the shows that might not see another season are "Arrested Development," a quirky comedy on News Corp.’s Fox, and "Jack & Bobby," the sentimental drama on Time Warner Inc.’s WB.

Die-hard fans have already mobilized, with letter-writing campaigns and online petitions. "Arrested Development" supporters have collected more than $1,400, through SaveOurBluths.com, and plan to send Fox hundreds of plastic bananas in protest. (The show’s Bluth family owns a frozen-banana stand.)

In the past, such efforts typically failed if the show’s ratings didn’t generate enough ad revenue to cover costs. But the cancellation calculus is starting to change. Fox’s "Family Guy," axed in 2002, is getting another chance on the air because sales of DVD collections of the show have been so hot. Indeed, fundamental shifts in how TV companies make money are starting to complicate cancellation decisions _ and small but dedicated groups of viewers are gaining newfound clout.

Of course, audience size still matters a lot in prime-time network television. But now other factors are influencing the cancellation decision. DVDs have become such a gold mine _ profit margins reach 50 percent _ that broadcasters are increasingly open to the idea of keeping a ratings-challenged show on the air, especially one with a fanatical core of fans, in order to generate more episodes to sell later on DVD. Other nascent income streams _ on-demand services, and downloadable episodes sold on the Internet like songs _ also are giving consumers more direct power in programming decisions.

So forget writing letters, says James Webster, senior associate dean at Northwestern University’s School of Communication. "Show networks how wide your wallet opens," he advises.

It worked for "Family Guy." Fans so far have snapped up about three million "Family Guy" box sets, which retail for $49.99. The irreverent cartoon ran on Fox for three seasons before getting the boot in 2002 due to poor ratings. It is set to return May 1 to a marquee slot on Sunday night.

"The explosion of this show on DVD proved to me that it had an enormous fan base that wanted more," says Gail Berman, Fox’s president of entertainment. Was she wrong to cancel it in the first place? "Changing her mind is, indeed, a woman’s prerogative," she says.

The DVD option wouldn’t have been possible without a 1993 Federal Communications Commission ruling relaxing ownership rules for TV shows. For the first time in decades, big broadcasters were allowed to have a financial stake in prime-time shows on their schedules. The ruling led to a flurry of mergers, with each of the Big Four networks now linked with a studio.

Before the ruling, back when networks could air only shows produced by outsiders, they cared only about selling ads. The show’s value in the aftermarket, such as profits from syndicated reruns, didn’t matter to them.

But now that networks own a portion or all of the shows they air, outside producers frequently grumble that network-owned shows have a different barometer of success. Network chiefs insist they don’t get paid to program with the financial interests of their sister studios in mind. But a network executive who can figure out how to keep a struggling show on the air to help the company’s studio make more money can win gold stars with the corporate bosses.

Networks don’t like talking about it for fear of appearing too cozy with their studio siblings, which is what led to the FCC’s now-abandoned ownership rules in the first place. But executives do concede privately that potential DVD sales are starting to play a bigger role in keeping shows on the air.

Of course, advertisers still care more about how many people see a program (and the advertising in it) than about how much they like the show. The equation eroded slightly in the 1980s, when advertisers began seeking out more young, urban viewers, a group considered more likely to try new products. So in some cases, a show with a smaller audience could command higher prices than a broad-based hit.

But cable TV and other leisure-time options kept pulling viewers away from network TV. Today, ABC’s "Desperate Housewives" is considered a blockbuster because it attracts an average of 20 million viewers _ ratings that would have led to cancellation in decades past. In the late 1960s, "The Beverly Hillbillies" attracted 60 million viewers a week.

In addition to "Arrested Development" and "Jack & Bobby," there are several other series facing oblivion. They include "Joan of Arcadia," a drama on Viacom Inc.’s CBS about a teenager who talks to God, and "Third Watch," a drama on General Electric Co.’s NBC about New York City firefighters and police officers, industry executives say. Meanwhile, despite wide speculation that "The West Wing" is in peril, NBC is in negotiations to bring the political series back in the fall. Although ratings have been dismal, the show still attracts one of broadcast TV’s most upscale audiences.

Despite the plastic bananas en route to Fox, the loudest rescue effort this spring is expected to be for "Star Trek: Enterprise," on Viacom’s UPN. The network announced Feb. 2 that it is ending the show’s voyage in May after four seasons.

"It’s far from over," vows Tim Brazeal, founder of SaveEnterprise.com. "Phase One of the attack," according to Mr. Brazeal, will include letter and e-mail campaigns. Phase Two is a 5,000-Trekkie march on UPN’s Los Angeles offices, set for Friday.

Letters and pickets don’t always fall on deaf ears. Rival networks sometimes take note and step in. UPN picked up "Buffy the Vampire Slayer" after Time Warner Inc.’s WB decided the show was too expensive. And DVD releases can still ensue: DVDs can hit the stores with only a handful of episodes, unlike reruns, which require about 100 episodes to form a programming package stations would buy. Citing fan petitions, Fox last month released a DVD set for "Wonderfalls," a short-lived drama about a shop clerk who hears voices.

Even if the zealous fans of "Star Trek: Enterprise" don’t get satisfaction by bringing the show back from the dead, they won’t need to despair completely: Viacom will release its first "Enterprise" DVD set in early May.