Are Britney Spears, Mariah Carey and Co. the X-factor to success?

Singing competition shows were supposedly about finding the best young talent. But that’s taken a back seat to a different imperative: Who’s got the biggest pop stars on the marquee?

That’s why Simon Cowell’s “X Factor,” which returns for Season 2 on Wednesday, dumped Paula Abdul and Nicole Scherzinger in favor of a pair with much better recent Billboard stats, Britney Spears and Demi Lovato.

And it’s why “Idol,” coming back in January, grabbed pop diva Mariah Carey for $18 million and is reportedly waving enormous checks at current hit-makers Nicki Minaj (Randy Jackson, a producer little known outside the music business when he signed on for “Idol’s” first season in 2002, will reportedly return as a judge after producers failed to make a deal with another current hit maker, Enrique Iglesias). NBC’s “The Voice,” which also returned this week, is keeping intact its star lineup of Christina Aguilera, Cee Lo Green, Adam Levine and Blake Shelton.

PHOTOS: Celebrity portraits by the Times

It’s an arms race, and the silo with the biggest missiles wins — or so TV executives may hope.

“Whenever they say, ‘It’s about the contestants’ and so forth, I don’t think it is,” said Brian Hughes, an analyst for ad firm Magna Global. “It’s about making a splash, right?”

Executives for “Idol,” “Factor” and “The Voice” declined to comment on the record. But a splash is indeed what these shows are aiming for. Producers say they’re in the business of finding pop stars — when Cowell left “Idol” a few seasons back, Fox executives repeatedly said it didn’t matter since the show was “all about the kids” — when they are in fact in the business of making hit TV shows.

Only two “Idol” winners out of the first 11 seasons have become durable pop stars: Kelly Clarkson and Carrie Underwood, who have together sold more than 26 million albums. But the record industry remains in the dumps. Overall, music sales have plummeted from $14.6 billion in 1999 to less than half that today, according to Forrester Research. Clarkson has moved so far beyond her “Idol” days that this summer she headlined a singing show on a rival network, ABC’s ill-received “Duets.”

The pileup of singing shows meanwhile has led to a roof-shaking battle for TV ratings. “Idol” was the No. 1 show on television for a record seven straight seasons, at its peak earning nearly $900 million in annual ad revenue, an astonishing sum for a single TV show. But last year’s premiere of “X Factor” took an immediate toll, and this spring, “Idol” saw double-digit audience declines across the board.

“The Voice’s” Season 3 premiere Monday opened to 12 million viewers, according to Nielsen. That was in line with the Season 1 start in April 2011 (11.8 million). Last February’s Season 2 premiere was artificially inflated (37.6 million) because it started after the Super Bowl. In any case, the lack of growth is another metric suggesting that singing shows have reached the limits of their popularity.

The intense competition among shows has tempers flaring. Cowell — whose $50-million-plus salary during his “Idol” days makes Carey’s paycheck seem puny — lashed out when NBC unexpectedly expanded this week’s season premiere of “The Voice” to a third night, Wednesday. NBC’s sudden move is meant to peel some of the luster off Cowell’s effort at a makeover for his show, which will unveil its first new episode in a head-to-head battle.

“It’s mean-spirited, and I hope and I pray that it backfires on them,” a seething Cowell said to reporters, adding that there should be a “gentlemen’s agreement” for the talent shows not to cannibalize one another’s audiences. (It’s a delicate situation, as Cowell is also the executive producer of NBC’s top summer hit, “America’s Got Talent.”)

The erosion of “Idol” and “X Factor’s” lower-than-expected ratings last year might be expected to reduce the allure for pop stars such as Carey and Spears. But the music industry now sees the immense value of sitting as a judge on a TV show.

Before Jennifer Lopez began her two-season stint on “Idol” in early 2011, her music career had stalled and she’d been dropped by her label, Epic Records. But the show’s exposure rejuvenated her stardom, yielding “On the Floor” — a mammoth dance-club hit and her first top 10 single in nearly a decade. The song was released just a few weeks after her debut as an “Idol” judge.

With singers making much less than they used to from record sales, the pressure to do TV becomes even more intense. Getting beamed onto people’s flat screens twice a week makes it much easier to sell concert tickets — an increasingly important source of income for performers these days.

“You’ve got to put butts in the seats at events,” said Scott Sternberg, a veteran reality producer who’s helped supervise shows for Abdul, Paula Zahn and others. “It’s all about branding.”

The real question is whether stacking the judges’ tables with platinum sellers will do anything to budge ratings. Many analysts see a glut of singing shows, and say that trend — combined with the advancing age of “Idol” — signal the beginning of the end for a genre that has dominated TV for about a decade.

“Idol” “is going to be entering its 12th season,” Hughes said. “There’s a limit to what you can do now to stem its decline. When a show is this age, it’s hard to reverse that, regardless of the judges.”

scott.collins@latimes.com