WASHINGTON - The nation's middle class, long a pillar of the U.S. economy and foundation of the American dream, has shrunk to the point where it no longer constitutes the majority of the adult population, according to a new major study.

The Pew Research Center report released Wednesday put in sharp relief the nation's increasing income divide, which is certain to be a central issue in the 2016 presidential race.

It also highlights how various economic and demographic forces have eroded long-held ideals about maintaining a strong, majority middle class.

Many analysts and policymakers regard the shift as worrisome for economic and social stability. Middle-income households have been the bedrock of consumer spending, and many liberals in particular view the declining middle as part of a troubling trend of skewed income gains among the nation's richest families.

Median-income voters, particularly non-college-educated men, are also at the core of billionaire Donald Trump's surprising surge in the Republican presidential campaign. His supporters' sense that their once-secure middle-class standing is in danger of slipping appears to be fueling much of the anger against the government and immigrant groups.

The tipping point for the middle class occurred over the last few years in the recovery from the Great Recession as the economy continued to reward highly educated workers, well-to-do investors, and those with technical skills.

Rapid growth of upper-income households, coupled with an increase in less-educated low earners, has driven the decline of the middle-income population to a hair below 50 percent of the total this year, Pew found. In 1971, the middle class accounted for 61 percent of the population, and it has been declining steadily since.

The Pew research found that the shares of upper-income and lower-income households grew in recent years as the middle shrank - with the higher-income tier growing more. In that sense, the nonpartisan group said, "the shift represents economic progress."

Pew defined middle class as households earning between two-thirds and twice the overall median income, after adjusting for household size.

A family of three, for example, would be considered middle income if its total annual income ranged from about $42,000 to $126,000.