In what TV pundits expect to be a tough contest for the most viewers, "The X Factor" returns to Fox television for a second season on Wednesday, after "The Voice" moves to the fall slot on NBC for a three-night season opener, starting on Monday.

Spears, 30, was tempted to join "The X Factor" by a reported $15 million salary after a 14-year singing career that made her the world's biggest pop phenomenon of the 2000s.

"X Factor" creator Simon Cowell is banking on her huge fan base and a strong curiosity factor to give his show a second chance with audiences after a disappointing first season in 2011 that attracted about 12.5 million viewers, and ended with the firing of two judges and the host.

"The Voice," which was watched by an average 15.6 million Americans last season, gets off to a rocking start on Monday with judges Christina Aguilera, Cee Lo Green, Adam Levine and Blake Shelton performing a cover of Rolling Stones hit "Start Me Up".

On Wednesday, the rival shows will compete head-to-head in a late programming move by NBC that Cowell condemned last week as a mean-spirited spoiling tactic.

"I think it's going to be a really tight race. At most, a big name gives a little pop in the ratings at the beginning," said James Hibberd, senior entertainment writer with Entertainment Weekly.

"But the celebrity judges are there to perform every bit as much as the people on stage, and their charisma and their wit need to be compelling to the audience," Hibberd said.

MEAN AND HARD TO PLEASE?

A survey by Yahoo! TV and Entertainment Weekly showed that 80 percent of those questioned were excited about Spears joining "X Factor". Some 59 percent of 18-35 female viewers said they would tune in to see how Spears, and fellow new judge, 20-year-old singer Demi Lovato, will fare.

Spears is little known outside her stage persona and choreographed sell-out concerts of hit songs like "Circus" and "Toxic".

But according to advance clips of early audition rounds, she has a sharp tongue for poor singers, and she is being promoted as competing with the sarcastic Cowell himself as the nasty judge on the show.

"She is surprisingly quite mean. She is difficult to please," Cowell told reporters last week.

But Hibberd noted that Spears was not well known for any off-stage wit.

"That's not to say she won't succeed. I am sure the first couple of episodes she will be packaged well and there will be some strong curiosity tune-in. But everything you know about Britney isn't really going to matter two weeks after the premiere," he said.

But Lovato said viewers were in for a surprise.

"I feel like the world hasn't really got a chance to see her (Britney's) personality over the last couple of years and now this is the perfect opportunity for people to see. She is very witty and funny and quirky," the former Disney Channel star told reporters.

Cowell is also aware that it takes more than new judges - however famous - to make a show that will distinguish itself in a saturated TV talent show field.

He says "X Factor" - which was seen by many critics last year as a virtual copycat of long-running "American Idol" on which he was a judge - has seen major tweaks this year.

It will have more of a reality show element, with lots of behind the scenes action, and later more focus on the judges and the mentoring process of the aspiring stars.

"You have got to make a different show to everybody else, otherwise they all blur into one," Cowell said.

"There is a lot more reality than we have ever shown before. If you have ever wondered what it's like to be on one of these shows, this show is a real glimpse of how stressful it is for the contestants," he said.

Fox is a unit of News Corp. and NBC is majority-owned by Comcast.

(Reporting By Jill Serjeant; editing by Patricia Reaney and Andrew Hay)

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