s the maid Marisa Ventura in the new film "Maid in Manhattan," Jennifer Lopez uses charm and hard work to vault the growing chasms between the rich, the poor and the getting-by (managing to snag the cute, rich guy along the way).

Popular culture has always embraced the notion of America as a nation of porous class lines. But as one of the most prosperous 20-year periods in American history ends, the question of just how important class has become has gained a new immediacy. Critics accuse President Bush of skewing his $64 billion, 10-year economic plan to help the rich, while he counters that opponents are engaging in un-American "class warfare."

Sociologists, of course, have argued for decades about how to define "social class." Yet even the latest research has produced contradictory conclusions.

When it comes to class, is income all that matters? Is your college alma mater important? Or knowing how to use a fingerbowl? And where does race fit in?

"There is a big academic debate on social class as opposed to income," said David B. Grusky, director of the Center for the Study of Inequality at Cornell and a professor of sociology. "There are sociologists who argue that social class is in decline in regard to lifestyle, consumption factors and politics as coherent, meaningful groups."

Searching for answers, Professor Grusky and Kim Weeden, an assistant professor of sociology at Cornell, turned to 30 years of data collected by the federal government and the National Opinion Research Center, affiliated with the University of Chicago. Together, the two surveys contained information on a representative group of about 760,000 Americans, from their political attitudes to their reading and television habits.

The professors concluded that lumping people into big groups like the "working" or "middle" class on the basis of their incomes ultimately had little to do with what they bought, what they watched or whom they voted for. Rather, cultural and political similarities are more likely to be found among people who are in the same profession or do the same type of work, reinforced first by educational training and then by work experiences.

Sociologists, for instance, are mostly politically liberal while economists are mostly conservative, they said.

Even big occupation groupings can hide differences, Professor Grusky said. Consider an issue like abortion. Among service workers, bartenders tended to support legalized abortion while cooks and cleaners tended to oppose it.

"Classes are as weak as they ever were," Professor Grusky said. "There is nothing shared in the big classes. Social scientists were always off the mark in talking about these big classes."

Paul W. Kingston, a professor of sociology at the University of Virginia and the author of "The Classless Society: Studies in Social Inequality" (Stanford University Press, 2000), agrees. He says that people who share a common economic position "do not significantly share distinct, life-defining experiences." And he further argues that economic inequality alone does not imply the existence of classes.

"The empirical issue is how would you recognize a class when you saw one," Professor Kingston said in an interview. "Does a blue-collar worker have a certain outlook, background, cultural disposition? The general impression is, there was a greater class structure 50 years ago. There is a lot of generational class mobility."

Professor Kingston says his research shows that habits in voting and tastes in music, television and recreation fail to correlate significantly with income. Class in the old-fashioned sense is arbitrary, he declares.

"Lots of people play golf, lots of people play tennis," he said, referring to two sports once seen as the province of the affluent. "Very few people go to the opera, and very few people discuss modern art. There are few of these things that speak to class."

The public's readiness to swallow Ms. Lopez's glamorous ascent from maid to mogul is just a small indicator of how deeply ingrained the idea of social mobility is in America. Charges of class warfare often fail to resonate because many people believe they have the chance to occupy the rich end of the income scale — despite the ever-widening income gap.

Robert Perrucci, a sociologist at Purdue University, explains this attitude by saying, "People accept inequality if they think there is opportunity." But he, like a number of other sociologists, maintains that class counts for more now than ever.

"Paul Kingston views class in cultural terms," Professor Perrucci said. "I look at it in economic terms. As a sociologist, I am concerned about the volatility of a society where 80 percent of the people are frozen out of possibilities. No one is saying we all have to be equal."

The average annual salary in America, expressed in 1998 dollars, went from $32,522 in 1970 to $35,864 in 1999. In the same period, according to Fortune magazine, the average real annual compensation of the top 100 C.E.O.'s went from $1.3 million — 40 times the pay of an average worker — to $37.5 million, or over 1,000 times the average worker's pay.

Professor Perrucci, along with Earl Wysong, at Indiana University of Kokomo and David W. Wright at Wichita State University, compared the incomes and occupations of 2,749 fathers and sons from the 1970's to the late 1990's. Their conclusion? That class mobility has decreased.

From the upper to the lower levels, the researchers found that sons retained the same levels of income and occupational prestige as their fathers. At the upper level, affluent sons gained prestigious positions — like doctors and lawyers — even more frequently than their fathers did 30 years ago.

"What has happened in the last 25 years is that a large segment of American society has become more vulnerable," said Professor Perrucci, who is co-author of "The New Class Society: Goodbye American Dream?" (Rowman & Littlefield, 2002) with Professor Wysong. "Twenty years ago, going to college was enough. Now, it has to be an elite school. The American dream is being sorely tested."

The new reality, he said, is a society in which one-fifth of Americans are privileged, with job security, high wages and strong skills. The other 80 percent belong to a "new working class," he said, that despite great variability within the group lacks the same security and high wages.

Professor Perrucci and his colleagues proposed four measures to determine where one lands on the class scale. They are: social capital (whom you know); credential capital (like where you received your degree); income or consumption capital; and investment capital (stocks and bonds). The last category is the one most affected by Mr. Bush's new economic program and tax laws.

Minorities, Professor Perrucci added, are far more likely than whites, especially white males, to lack elite educational credentials and social capital.

Although the black middle class roughly doubled in the last 20 years, about 30 percent of blacks (compared with 8 percent of whites) are poor by government standards. Numerous studies also show that continued discrimination in housing and jobs stymies black economic mobility, as does the perception that minorities have different values and behavior from whites.

Erik Olin Wright, a sociology professor at the University of Wisconsin in Madison, argues that race is one of the factors that mute the expression of class conflict.

"To say there's no class is to imply that the workers at Enron and the owners at Enron have no built-in conflict, no different outcomes," said Professor Wright, who is also the author of "Class Counts: Comparative Studies in Class Analysis" (Cambridge University Press, 1997).

But it is "simple-minded," he said, to argue, as some sociologists have done, that the absence of open hostilities means there are no real social classes with similar interests.

Race divisions, for example, could discourage people from similar classes to come together and push for better health care or schools, Professor Wright said. Or people may be too "demoralized and resigned," he added, to organize movements that pit working people against rich people.

"It may be the triumph of one class," he said, "one class that is so hegemonic that people feel defeated."