To those civilians who merely live and work in buildings, architecture may appear to be a genteel profession dominated by people in cool eyeglasses and black clothing. Little do they know of the furious clashes raging between the Modernists and the New Urbanists, an ideological rift every bit as bitter and unbridgeable as America's Red State/Blue State divide.

After Hurricane Katrina, when many Americans had harsh words for the Bush administration, FEMA and the Army Corps of Engineers, the Modernists trained their ire on the New Urbanists and their rebuilding proposals. The utopian planning geeks were described as a cult bent on imposing "cartoon cities" and "smiley-face architecture" on a gullible public. The New Urbanists shot back that the Modernists would have people invest their life savings in "experiments."

Now, the New Urbanists, about 1,500 of them, have arrived in Philadelphia for the annual Congress for the New Urbanism. It is one thing for them to meet in this gritty redoubt of the Old Urbanism. But the New Urbanists also have stormed the great citadel of Modernism: the PSFS Building, the first American example of International Style architecture, now a Loews Hotel.

"You should see people's reactions," Steve Filmanowicz, the group's communications director, said with a chuckle. "There are things about it that people definitely did not expect."

Once the shock wears off, he said, the conferees have positive things to say about the Market Street tower, completed in 1932 by architects George Howe and William Lescaze.

"We think it's a nice building, with nice materials," Filmanowicz said. "This is the kind of Modernism that New Urbanists can appreciate."

Don't be surprised if there are holdouts.

"What's missing from most Modernist buildings is the graspable detail, ornament," said Ray Gindroz, a Pittsburgh architect who helped develop the Katrina Cottage, a house kit for storm victims. The PSFS Building has style, but not a shred of applied ornament.

New Urbanism began in the 1980s as a reaction to the wasteful land practices of suburban sprawl. Adherents advocated a return to compact towns built around a walkable commercial center. But the movement soon became conjoined with a philosophy that rejects Modernist functionalism in favor of traditional building types heavy with decorative elements.

New Urbanist gurus Andres Duany and Elizabeth Plater-Zyberk rose to fame for their planning work at Seaside, Fla., the setting for The Truman Show. Although they design contemporary buildings, and Plater-Zyberk's father once worked in the PSFS tower, they often criticize modern stararchitects for creating ego-driven works that ignore their urban context.

The architectural choices are certainly familiar to Philadelphians, who continually struggle to find ways to accommodate modern materials and skyscrapers into an overwhelmingly low-rise, red-brick landscape.

After Milwaukee's highway-fighting mayor, John Norquist, became the congress' president in 2004, he began to nudge the group away from the style wars and back to substantive issues such as mass transit. He pitches New Urbanism's compact towns as a way to reduce automobile use and help the environment.

Modernist critics aren't buying it. When Duany and the New Urbanists organized a Gulf Coast planning charrette five weeks after Katrina, the Modernists went on the attack. Reed Kroloff, then architecture dean at Tulane University, warned the neotraditionalists would "Disney-fy" New Orleans.

California architect Eric Owen Moss' remarks were more incendiary: The New Urbanists' agenda appeals to an "anachronistic Mississippi that yearns for the good old days of the Old South . . . and each person knew his or her own role."

Other New Urbanist critics hail from the right. Catesby Leigh of American Enterprise magazine argued that sprawl and America go together like stars and stripes. "The New Urbanists," Leigh complained, "overlook the fact that suburbia has satisfied the aspiration of countless Americans to ownership of a freestanding house on a spacious green patch outside the increasingly ill-governed city."

Fortunately, if the rhetoric gets too hot for the New Urbanists, they can always take the PSFS' famous escalator down to the streets of Philadelphia, where the old, new, staunchly traditional and wildly modern manage to coexist quite nicely.