WASHINGTON – The middle class is receiving less of America’s total income, declining to its smallest share in decades as median wages stagnate in the economic doldrums and wealth concentrates at the top.

A study released Wednesday by the Pew Research Center highlights diminished hopes, too, for the roughly 50 percent of adults defined as middle class, with household incomes ranging from $39,000 to $118,000. The report describes this mid-tier group as suffering its “worst decade in modern history,” having fallen backward in income for the first time since the end of World War II.

Three years after the recession technically ended, middle class Americans are still feeling the economic pinch, with most saying they have been forced to reduce spending in the past year. And fewer now believe that hard work will allow them to get ahead in life. Families are now more likely to say their children’s economic future will be the same or worse than their own.

In all, 85 percent of middle class Americans say it is more difficult now than a decade ago to maintain their standard of living. Some 62 percent say a lot of the blame lies with Congress. A slight majority say a lot lies with banks and other financial institutions. Just 8 percent blame the middle class itself.

“The job market is changing, our living standards are falling in the middle, and middle-income parents are now afraid that their children will be worse off than they are,” says Timothy Smeeding, a University of Wisconsin-Madison economics professor who specializes in income inequality.

He said that many middle-income families have taken a big hit in the past decade as health care costs increase, mid-wage jobs disappear due to automation and outsourcing and college tuition mounts for those seeking to build credentials to get better work. In the meantime, more-affluent families have fared better in net worth because they are less dependent than lower-income groups on home property values, which remain shriveled after the housing bust. Wealthier Americans are more likely to be invested in the stock market, which as a whole has been quicker to recover from the downturn.

“These are the disaffected middle class who work hard and play by the rules of society, but increasingly see their situation declining by forces beyond their control,” Smeeding said in an interview. “No matter who is president, the climb back up for the middle class and the recovery will be slow and often painful.”

The Pew study is just the latest indicator of a long-term trend of widening U.S. income inequality. The Census Bureau reported last year that income fell for the wealthiest — down 1.2 percent to $180,810 for the top 5 percent of households. But the bottom fifth of households — those making $20,000 or less — saw incomes decline 4 percent.

The new study reviewed 2010 data from the Census Bureau and Federal Reserve, defining “middle class” as the tier of adults whose household income falls between two-thirds and double the national median income, or $39,418 to $118,255 in 2010 for a family of three. By this definition, “middle class” makes up about 51 percent of U.S. adults, down from 61 percent in 1971.

In 1970, the share of U.S. income that went to the middle class was 62 percent, while wealthier Americans received just 29 percent. But by 2010, the middle class garnered 45 percent of the nation’s income, tying a low first reached in 2006, compared to 46 percent for upper-income Americans.

Since 2000, the median income for America’s middle class has fallen from $72,956 to $69,487.

“The notion that the middle class always enjoys a rising standard of living is a big part of America’s sense of itself. And in modern times, it’s always been true — until now,” said Paul Taylor, executive vice president of the Pew Research Center.

“Middle class Americans still have faith in the future — their own, their children’s, the country’s. But their outlook is not as rosy now as it was before the recession began,” he added.