Last week Zogby International released a poll which found that just 50 percent of the American public believes that the “American Dream” is still attainable. This latest vague, but important, measure of optimism compares to 68 percent in November 2008 – perhaps not coincidentally, the same month that Americans took a gamble on “Hope.”

Loss of confidence that the future will be brighter, or even as bright as the present, is a disturbing sign. But new data released by the Census Bureau add to those concerns. Economic well-being is generally the most important factor in whether people are optimistic or pessimistic about their futures. The new Census data show a chasm, not a gap, between the economic fortunes of white Americans as compared to blacks and Hispanics. But even this breakdown may not tell the full story, as “white wealth” is skewed by the fact that the super-rich in this country are, by and large, white.

Perception – the fact that a mere half the population believes the American Dream is still within their grasps – and reality – the fact that the fastest growing segments of our population are falling farther and farther behind – pose a potentially lethal threat to our unity as a nation. Unlike traditional nations which are bound by common bloodlines, the United States has thrived as a diverse nation based on (for the lack of a better term) the American Dream.

What has united our diverse population is the widespread belief that the system is fair and just and affords everyone who is willing to work hard and play by the rules a chance to succeed. We know that the nation has not always lived up to its ideals in all respects, but on balance Americans have believed that freedom, fairness and opportunity were their birthright.

As mass immigration brings in millions of people who have no historic investment in this nation and who grow frustrated and alienated because they are failing to achieve their American Dream, the essential glue that binds us as a nation may quickly dissolve. When people cease believing that the playing field is level (or even close to level) and that working hard and playing by the rules will get them ahead, they inevitably quit playing by the rules. Add to that a sense of estrangement among segments of our population that are not connected by language, culture or history, and the divisions may become irreconcilable.

There are many factors that are contributing to the sense and reality that America is becoming a less equitable society, and fixing our broken immigration system would not be a magic cure-all. A system that has concentrated an ever larger share of wealth in the hands of a very few and their growing disengagement from the bottom 95 percent of us cannot be underestimated. Nevertheless, mass immigration is a critical exacerbating factor and could prove to be the match that ignites a social and economic tinderbox.