By GLENN GAMBOA
Staff Writer

 For Madonna, necessity is the mother of "Re - Invention."

 Super - savvy culture vulture that she is, the Material Mom knows that if she doesn't re - assert her relevance soon, she could quickly become a fringe best known for writing children's books and being Britney's gal - pal.

 On the heels of disappointing sales for her "American Life" album and a hostile reception to her last movie, "Swept Away," the 45 - year - old entertainer has her back against the wall for the first time in her career.

 "The Re - Invention Tour" is her way of proving she is not ready to retire to the London mansion with hubby Guy Ritchie and the kids just yet. At Madison Square Garden last night, the first of eight sold - out shows in New York in the next two weeks, she definitely made that point.

 Many sing better. Others write better songs. But no one performs better than Madonna especially when she has something to prove.

 In the nearly two - hour set, Madonna takes the audience on a whirlwind tour through her 20 - year career. Some songs get shaken up the disco jam "Deeper and Deeper" gets jazzy, "Like a Prayer" gets an electro - country twang and "Material Girl" becomes a pop - punk rave - up. But what is even more impressive is how her elaborate performance art pieces enhance many of the songs. The athletic swinging of her dancers during "Bedtime Story" provides the song a grace that it never would have seen in a straight performance. The intricate moves of her 16 - member dance troupe turned "Into the Groove" into a powerful dance piece instead of simply a dance - pop trifle. "Papa Don't Preach" was filled with playfulness and innocence, even including a ring - around - the - rosie dance.

 This is a side of Madonna that she hasn't shown very often, the one that has fun, the one that enjoys the roar of the crowd. She offered genuine appreciation for the cheers much like her decision to bring back songs from her past that she has tired of.

 Making peace with her past doesn't mean she's ready to give up on the interests of her present. Kabbala is present in the Hebrew letters that swirl on the big screens behind her and she even sports a t - shirt that says "Kabbalists Do It Better." Her anti - war, anti - Bush beliefs are clearly on display during "American Life," as well as her overwhelming endorsement of Michael Moore's "Fahrenheit 9/11," which she said had her in tears. With this tour, Madonna can give the song her full support unlike last year, when she yanked the video to avoid controversy about complaining about Middle East policies in the middle of the Iraqi war. She was rightfully worried about getting Dixie - Chicked if she proceeded, but as it turned out, her album was basically blacklisted anyway.

 If anyone could have waged a successful media campaign to get her anti - war, pro - troops point across, it would be Team Madonna. The Material Mom, however, plays things a little safer, which is also evident on "The Re - Invention Tour."

 When Madonna dedicated her ballad "Crazy For You" to all her fans who have "stuck with me through thick and thin for the past 20 years," she short - circuited all the critiques of her 2001 "Drowned World" tour, which was high on drama and production but low on fun and any sort of crowd interaction.

 Madonna's latest reinvention may be her best one yet. She has not only become a champion of the underdogs but somehow an underdog herself. And as long as she offers amazing performances like this one, she will have an army of fans backing her up.