There have been signs of the shift for at least a couple of years, but now it's official: the tsunami of immigrants that has flooded the U.S. for the past four decades, and inspired a Noah's Ark mentality amongst xenophobes, Nativists and know-nothings, is receding.

If the anti-immigration alarmism in this country were based solely on the issue of undocumented aliens, as many conservatives claim, you'd be hearing hallelujahs and shouts of joy from the Right. After all, there's not much point in wasting American tax dollars on a wall that will only keep Mexicans from leaving the U.S. Problem solved, right?

But most Republicans have been conspicuously quiet. Why? Because a report issued this week by the Pew Center showing that the total number of Mexican non-citizens going out of the U.S. has exceeded the number coming in for the first time since the Great Depression, robs them of a sure-fire way to rally the faithful and drum up support from their mostly white base. Now all those Super PAC election ads showing brown people swarming northward across the Rio Grande like zombies will have to be shelved; images of brown people swarming back to Mexico simply doesn't pack the same ideological punch.

According to the PEW report, the Mexican-born population in the U.S. peaked at 12.6 million in 2007 and has declined to 12 million since then. The number of undocumented Mexican in the U.S. fell from 7 million in 2007 to 6.1 million in 2011. Meanwhile, arrests of immigrants trying to cross into the U.S. from Mexico have dropped significantly in recent years; and crime rates in most Southern border states has actually declined.

The boom-and-bust cycle that has tied immigration to economic prosperity is a recurring theme in U.S. history - from African slaves to Chinese railroad workers to Europeans who settled the Western frontiers and toiled in the factories of the Industrial Revolution. More recently, immigrants from Latin America have played a crucial role in filling low-skilled jobs in agriculture, construction and the service industry, which long ago replaced manufacturing as the biggest sector of the U.S. economy. Without the robust immigration rates of the past few decades, the United States would have already joined Japan and Europe in negative population growth, which is linked to falling productivity, waning international influence, and lopsided dependency ratios as a shrinking pool of younger workers support a growing ocean of retirees.

Most demographers believe that the new North to South net immigration outflow is likely to be permanent, but the demographic trends toward a more multicultural America are equally indelible. The real significance of the PEW report may be in the implications for the Latinos, both immigrant and native-born, who remain here. The National Institute for Latino policy estimates that 12.2 million Hispanics will vote in 2012, up from 9.7 million in 2008. Pundits predict that the Hispanic vote could be the determining factor in several states this year, but it's clear that Latino influence in future elections - and their percentage of the total U.S. population - will only grow.

Earlier this month, I attended a round table discussion in Washington D.C. ,co-sponsored by the National Journal and the University of Phoenix, around the theme "The Next America: How Demography Shapes the National Agenda." The attendees, who included prominent journalists, educators, and a former head of the U.S. Census Bureau, agreed that the critical issues that impact minorities in the U.S. - health, employment, education - are the same ones that impact all Americans, because the interests of the emerging multicultural majority and the interests of the nation are intrinsically intertwined. The future of America is multicultural by definition, and neither the natural ebb and flow of immigrants across the U.S. -Mexican border, nor misguided laws designed to stop it, will change that.

Perhaps the time has come for a new definition of American Exceptionalism that not only acknowledges America's mission to spread and uphold freedom and liberty, but also embraces and celebrates our unique ethnic and racial diversity as an advantage over nations with more monolithic cultures. America's emerging multicultural majority is a window and bridge to other countries around the world, opening the door to a plethora of social, political and economic benefits. Without the industry, energy and patriotism of immigrants and their native-born sons and daughters, the City on the Hill would have lost its shine a long time ago.