By Jill Serjeant

LOS ANGELES - Ellen DeGeneres makes her much-anticipated debut on "American Idol" on Tuesday, shaking up the dynamics on the show's judging panel and auditioning her own talents for the biggest TV audience in the United States.

The arrival of the popular talk show host and comic is expected to boost viewership on the Fox television network's aging singing contest, now in its 9th season and facing challenges to its coveted spot as the No. 1 show on US TV.

The biggest question is, can Ellen get viewers to stay?

DeGeneres, 52, takes the seat held by Paula Abdul, who quit last summer in a contract renewal dispute, and "Idol" fans and TV critics remain passionately divided over her replacement.

"Ellen brings a huge fan base and people will be curious to see how she does. But if she performs poorly, that is bad for the show's long-term health," said Entertainment Weekly's Michael Slezak.

The pop culture phenomenon has ruled TV ratings for six years, commanding top dollar from advertisers and producing Grammy-winning stars like Carrie Underwood and Kelly Clarkson.

But audiences have slipped from a 2006 high of about 30.8 million per each twice-weekly episode. 2010 viewership is currently down three percent at an average 26.8 million compared to 2009's opening weeks, according to Fox.

The series also faces a challenge this year from rival network CBS's popular crime series "NCIS", and for the first time in years the music industry's Grammy Awards in late January drew a bigger TV audience than "Idol".

DeGeneres lacks the music industry credentials of fellow judges, Randy Jackson, a record producer; songwriter Kara DioGuardi; and sharp-tongued Simon Cowell, who has his own record label and two other TV talent shows.

But the witty talk show host, who has said she found Cowell "even meaner" than she expected, says she aims to be the voice of the viewer at home.

"I'm someone who knows what it's like to be on stage and entertain. So I know what it takes. It's not like I don't listen to music constantly," she told TV news journalist Katie Couric in a TV interview last week.

Judging the judges

Although "American Idol" is primarily about the search for the next pop superstar, the colourful judges are arguably as much part of the show's success as the hopefuls before them.

"The judges and their chemistry is integral to 'Idol's success. They provide the dramatic tension," said Todd Gold, managing editor of Fancast.com.

With Cowell leaving "Idol" in May to launch his own show on Fox in 2011, the success of DeGeneres is crucial, said Brian Mansfield, who runs the USA Today Idol Chatter blog.

"If Ellen works, it puts (producers) in a much better position when they replace Simon. The dynamic is the thing I am curious to watch. I think some people will miss Paula's 'camp mom' spirit," Mansfield said.

Fan emotions are running high. "With Paula leaving I was about to give up on 'Idol,' but then I find out Ellen was on board?! Perfect! The show will be better than ever!" wrote xanadude in a posting on the official "American Idol" online forum.

However Slezak believes the real draw of the show is the wannabe stars and their ability to turn out future hits.

"As much as we like to talk about the judges, the show lives or dies by the contestants and how they do once they are off the show," Slezak said.