As we slink into the sweltering core of summer 2002, the most surprising development on the pop charts is a bit of trivia that could be a sign of major changes to come or just a heat mirage. Right now, the biggest-selling artists of the season are Eminem and Nelly, two rappers who come from the Midwest, the region of the country least likely to lead the tattered hip-hop nation. (Yes, underground rap is thrilling these days, but in the real world, rap sales are hardly booming the way they were a few years back.) What’s more, Detroit’s Eminem and St. Louis’s Nelly fill opposite sides of the same role: the crude-talking, low-pants-hanging, hip-hop Everyman.

For five weeks, Eminem occupied the top of the Billboard album chart by playing the bad-ass side of that Everyman on The Eminem Show (Interscope), the aptly titled third installment of his ongoing public psychodrama. "So many lives I touch, so much anger aimed in no particular direction," he raps on the opening cut. It’s a familiar theme, but Eminem is now out to seize the bully pulpit. Backed by ostentatious rock production (and a first-ever lyric sheet), he explains how his rise has everything to do with the one subject he’s always sidestepped: race. "White America! I could be one of your kids," he intones. "Let’s do the math, if I was black, I would’ve sold half."

A sharp observation, no question, but last week the math didn’t add up. Over that seven-day stretch, Nelly’s second album, Nellyville (Universal), scored almost three-quarters of a million sales, more than twice as many as The Eminem Show did in the same period. On the surface, that seems a fluke. Whereas the originality of Eminem’s sound and fury was certified two albums ago, Nelly offers little more than a party groove whose originality is still hotly debated.

The patent for Nelly’s popcorn calisthenics was first submitted two years ago on "(Hot S***) Country Grammar," a smash single whose irresistible bounce lifted the 23-year-old rapper from his status as a regional sensation into one of the biggest-selling artists of the decade. It became the title track to his major-label debut, an album that went on to sell eight million copies by rebounding the bounce off supporting tracks like the self-explanatory "Ride wit Me" and the Spanglish-sparked "E.I." A few weeks ago, Nelly made the first move to repeat that achievement by releasing the advance single for Nellyville, a sizzling track with a simple answer for the summer swelter: "It’s getting hot in here/So take off all your clothes." It doesn’t quite beat the bounce of "(Hot S***) Country Grammar," but how can you improve on a nursery rhyme? "Down, down, baby, I can do karate!" chanted my three-year-old after day care one evening, repeating the rhyme every day for months afterward with no loss of pleasure.

No wonder that Nelly was able to do the same for a nation of children at heart. All he did was transpose a similar double-dutch rhyme (I’m going down, down, baby . . . cocked, ready to love you") into the playa’s realm of gun-toting, pot-smoking, gear-flashing hedonism, thus making the thrill of unfettered hip-hop decadence accessible to anyone who’s ever flossed on a jungle gym. On the new, clothes-shedding single, the sinewy rapper hits the same sing-song tone, exchanging the gat waving for extra back shaking and the double-dutch beat for a sample of Chuck Brown’s early-’80s go-go classic, "Bustin’ Loose." At once plusher and looser than "Country Grammar," this new one is hot stuff. Yes, it’s doomed to fall short of its predecessor, if only because it came second. But to make the most of that inevitable outcome, Nelly has added one more twist — an extra "r" to the song title, turning "Hot in Here" into "Hot in Herre." The reason is simple but crucial: that’s the way they say it in St. Louis.

St. Louis is to Nelly what white skin is to Eminem — a factor he had to overcome before he could use it to his advantage. Like so many old Midwestern industrial towns, the city proper has experienced a collapse — it’s now smaller than Kansas City, which lies just across the state, or Oklahoma City, which lies just across the empty plains. But even if St. Louis were thriving, major record companies wouldn’t have touched it until very recently.

As accomplished New York underground rapper Talib Kweli once told me, "Hip-hop any place else besides New York is an import." Kweli was referring to Cincinnati, the home town of his DJ, Hi-Tek, but he could have been talking about any Midwestern town. "Cincinnati is very depressing to me," he explained. "Out there, hip-hop culture doesn’t thrive like it does in some other places in the world. It’s like, it’s a hard place to be, in the middle of America. I mean, the first time I ever got arrested was in Cincinnati, for, like, talking back to a cop — some shit in New York that I do every day, you know what I’m saying? And he was, like, he threw me on the ground like it wasn’t nobody’s business. It’s just the way they got people living out there. It’s just real oppressive [and] it’s hard to break out of that mentality. A lot of people in the middle of the country never go nowhere."

But as Kweli was willing to concede, this is an outsider’s perspective. Those who actually live in the Midwest have long learned to make something from too much of nothing (as Minnesota refugee Bob Dylan put it), and Nelly has done the same in hyping St. Louis. A listen to Country Grammar makes it clear that this hype is at least partly self-invention. His sing-song patter slips all over the map. He throws down some smooth West Coast synth whine mixed with a lot of Dirty South bounce, plus a dash of Bone Thugs-N-Harmony’s sentimental rap harmonizing to thicken the stew. In terms of both politics and culture, Missouri has always been a border state, so the direct influence of the No Limit and Cash Money crews, the entrepreneurial Southern gangsta rappers who busted open rap’s bi-coastal lockdown, is surely real. But with the possible exception of Mystikal, who broke free of No Limit as soon as he could, no hot-selling New Orleans rapper has made this bounce as widely appealing as Nelly has — his version lacks the ugly menace, the sense of exclusion projected by most Dirty South artists. And the biggest pleasure of Nellyville is that the rapper dares continue down that path, opening up his music even more.

At first, the generic ease of the disc makes Nellyville something of a disappointment compared with the quirky jolt of Country Grammar, or even the raw rush of Free City (all are on Universal), Nelly’s quickie disc with his St. Louis disciples, the St. Lunatics. But within a couple spins, the simple hooks click on almost every track, from the blaxploitation funk of "Pimp Juice" to the tricky intertwined vocals of "On the Grind" to the gentle swing of "Splurge," which lives up to its title by laying on the horns. Sure, conspicuous consumption is a questionable philosophy for the good life, and Nellyville bogs down toward the end as the rapper feels compelled to play hard (and answer his most notable detractor, KRS-One). But beyond the high living and tough talking is a subject that comes on far stronger than it did on Country Grammar: hot sex. Even the track featuring Nelly’s newest pal, Justin Timberlake of ’N Sync, gets nasty. And as raw hedonism, his aural porn demonstrates more lift and less contempt than the typical Internet girlie site — or the typical Snoop Dogg album.

Credit this lightness to two elements. First is Nelly’s underappreciated vocal prowess. As a rapper, he almost never surprises, and as a singer his vocal flights are fairly pedestrian, but he straddles the two realms with an ease that’s unparalleled. This achievement has nothing to do with his St. Louis bounce, but it sure helps it ride smoother. The second element is the community of producers and fellow rappers that this hardscrabble child of divorce has brought together in his native city.

That community is honored most lovingly on the opening title track. Whereas Eminem opens his Show by unmasking his complex relationship to "White America" — his debts and his resentments — Nelly kicks off Nellyville by turning his home-town boosterism into a fantasy of inclusion. In the grand, idyllic tradition of the Staple Singers, Big Daddy Kane, and Coolio, Nelly sings about heaven made manifest on earth. In Nellyville, "all newborns get half a mil," and the promise of 40 acres and a mule turns into "40 acres and a pool." "What happens in the Midwest is you’ve got the best of all worlds," continued Kweli, plowing through the apparent contradiction. "You’ve got the West Coast influence, plus the East Coast influence, plus your own shit, plus the South shit. Go back, think about the O’Jays and the Isleys. I mean, like, there’s so much incredible music that comes out of the Midwest. . . . I mean, there’s a reason that Motown was in Detroit."

So maybe there’s also a reason Nelly is in St. Louis. For starters, all those empty lots sure make the pool digging easier.