Last week, American Idol was encumbered with Disco Night.

Though the clock was turned back a decade further this week, the songwriting mojo got cranked way up.

Selecting music popularized by the Rat Pack in the early 1960s — though music mainly written in the ’40s and ’50s — the show offered the rare opportunity to really give the singers something they could sink their collective chops into. Plus, they could dress sharp.

Adding to the prestige was the announcement of a “mystery mentor.” For some reason, the talented Jamie Foxx was hauled out for the occasion, despite being completely unaffiliated with the genre — perhaps it had more to do with the fact he had a movie opening the same week (“The Soloist”).

(I wondered why the producers didn’t instead opt for Don Cheadle, since he actually played Sammy Davis Jr. in that “Rat Pack” movie.)

Mercifully, the Oscar-winning actor proved a dedicated coach. And it was tough to argue with his results.

Undoubtedly, this lineup of five contestants offered the best collective vocal display at this stage of the competition in the show’s history. There were no disasters and at least three major victories.

Kris Allen confirmed he wasn’t just a John Mayer knockoff with a terrific version of “The Way You Look Tonight.” Allen offered loads of charisma and range, and he simply looked comfy doing a BIG SONG. Regardless of the fact he possesses the weakest pure vocal facility, he’s likely the most commercial artist of the bunch. Ever the dark horse, he still holds a shot at winning the whole thing.

Recently turned 17-year-old Allison Iraheta tackled the difficult “Someone to Watch Over Me.” Appearing classier than she ever has before — especially compared to judge Paula Abdul, whose dress made her look like a half-wrapped birthday present — Iraheta seamlessly blended her rock/blues tonality into the torch song. Judge Simon Cowell astutely questioned her overall motivation, though. While he assumed it had something to do with her belief that she could win, I perceived it as more her lacking killer instinct. To paraphrase an overused NBA cliche, it seems like “the other guy just wanted it more.” That refers to ALL the other guys she’s up against. Iraheta definitely needs to aim for the conceptual jugular if she hopes to topple the other competitors.

On paper, Matt Giraud was poised to be the most innately comfortable with the jazzy standards domain. After all, he already owns the right hats. While he offered a respectable take on the ever-popular “My Funny Valentine” (with its odd “are you smart?” lyrics), Giraud just didn’t manifest enough vocal magic. If he were in the actual Rat Pack, he would probably be booked at Bally’s instead of the Bellagio.

Finally, Danny Gokey showed why he’s a perennial pick to win Idol. Spurred on by some terrific advice from Foxx in which the star literally invaded the singer’s space, Gokey’s full potential was unleashed during “Come Rain or Come Shine.” He nailed the swagger, the dynamics, the tone. This was the first time since the round of 13 that I began to believe Gokey could eventually be crowned winner.

The other media-appointed frontrunner, didn’t fare quite as well. Adam Lambert had the tailored outfit, but that was about all that came across as Rat Packy. In fact, he seemed a little more Brat Packy with his theatrical rendition of “Feeling Good.” While there were several choice vocal “moments,” the whole performance exhibited the vibe of something you’d see at a drag show in the ’80s.

Bottom three prediction: Matt Giraud, Allison Iraheta, Adam Lambert.

Losers: Matt Giraud.