The two Davids square off tonight, having already knocked the increasingly predictable

American Idol

Goliath down a few pegs.

Sure, the finish is likely to produce a come-from-behind winner, but he passed the favorite weeks ago, and viewers, feeling the inevitable, have been turning away.

David Archuleta, 17, favorite in the most-likely-wholesome-first-boyfriend-for-Miley-Cyrus competition (anyone over 15 who can even imagine him having sex has serious problems), is presenting a brave and extremely lovable face.

But David Cook, 26, has replaced Archuleta as the big favorite to win Idol Season Seven, when the two caterwaul for all the cookies beginning at 8 p.m. on Fox29.

Besides general stagecraft that overwhelms the doe-eyed teen, Cook has craftily mixed a few days' beard growth with a new, adult haircut, to hone his initial cute boy appearance into a more dangerous and world-weary look, all the better to snag the votes of everybody over 15.performs.

Oh, and he sings better, too, perhaps with not as wondrous a voice, but with way many more vocal moves that serve him in interpreting the variety of songs required in the contest.

Not that it has made any difference in the downward direction of Idol. The 21.8 million people who watched the performance show last week made up Idol's smallest Tuesday audience since its early days, more than 20 percent below this year's average. Overall, the show is off a more modest 5.6 percent over last year, but it's the second consecutive season-to-season decline.

Fox is far from crying in its champagne, however. The Idol audience may be aging, as fad-fascinated young folks head for The Hills (and elsewhere), but Idol still swamps the ratings of TV's No. 2-rated show, ABC's fogey-friendly Dancing With the Stars, by more than 40 percent.

Tonight's show is only an hour, which is a blessing, and Fox has already announced that next season's Wednesday results show will slim down to a half hour most weeks, which may help audiences avoid that uncomfortable bloated feeling.

Cook, always described as "a rocker" on the show where Neil Diamond passes for heavy metal, fits the classic Idol pattern of a winning contestant who has seemed to get progressively better, with a few backward stutter-steps, each week.

The Internet odds favoring him bounced around yesterday, getting as high as 2 to 5 (bet $25 to win $10), on www.bodoglife.com, one online bookie, with Archuleta backers winning $42.50 for $25, if their guy comes in.

As soon as he opened his mouth on the show, Archuleta seemed the heir apparent to last year's 17-year-old victor, Jordin Sparks. The usually stern Simon Cowell, most likely overcome by a brain invasion of dollar signs as he envisioned signing the next 'tween heartthrob, let his jaw drop as that big voice emerged from that sweet face at the auditions.

But Cowell's tune changed on the March 11 Idol, when he told Cook, "If this show remains a talent competition rather than a popularity competition, you actually could win this entire show."

Then Cowell might sign two million-dollar babies to his label, because the little girls are going to flock Archuleta no matter what, and it's possible that people who actually like music might pay for Cook's output.

The last successful Idol "rocker," Chris Daughtry, who finished only fourth in Idol's fifth season, has sold more than three million copies of his self-titled post-show album. That ranks just behind discs released by the show's two most successful winners, Carrie Underwood and Kelly Clarkson.

If Cook does close the deal, and is announced the winner tomorrow night, it could mean a boost for the Idol music machine, even as its television branch continues to decline.

Jonathan Storm:

Television

American Idol

Tonight at 8 on Fox29

To comment on this article, go to: http://go.philly.com/askstorm. Contact television critic Jonathan Storm at 215-854-5618 or jstorm@phillynews.com. Read his recent work at http://go.philly.com/jonathanstorm.