(Bloomberg News) The middle class is shrinking and is now barely a majority in the U.S., underscoring the challenge President Barack Obama and Mitt Romney face as they argue over who can best protect the wealth of struggling Americans.

The proportion of middle-income earners, or those making from $39,000 to $118,000 for a family of three, narrowed over the decade as the housing-market crash erased more than 20 years of gains in wealth and average incomes fell for the first time since World War II, according to a Pew Research Center report released today. The middle class shrank to 51 percent of adults in 2011 from 61 percent in 1971.

"We think of ourselves and pride ourselves as being a middle-class society," said Paul Taylor, the center's executive vice president and the editor of the report, in an interview. "But there's a sense that the middle class is getting smaller. It's hollowing out."

Middle-class people are reluctant to lay much of the blame on Obama, with Congress, banks, corporations, the Bush administration and overseas competition more commonly cited as the cause of their problems, according to the survey. More than half, 52 percent, say Obama's policies would aid the middle class, a 10-percentage-point advantage over Romney.

'Lost Decade'

The report, "The Lost Decade of the Middle Class," highlights the effects of the longest recession since the Great Depression that left 12.8 million currently unemployed and pushed more into poverty. The plight of middle-income Americans in an economy that has yet to fully rebound more than three years after the recession ended is a central theme of the presidential race.

"This is the first decade in modern history where median household income hasn't returned to an earlier peak," Taylor said in a conference call. "Just about everyone has taken a hit."

Romney, who co-founded private-equity firm Bain Capital and served as Massachusetts' governor, has faulted Obama for failing to spur a stronger recovery. Obama has cast himself as a defender of the middle class and criticized his opponent, who has an estimated net worth of as much as $250 million, for seeking to cut taxes on higher-income Americans.

Obama's pitch may resonate with middle-income voters, the Pew poll indicates. Republicans favor the rich, according to 62 percent of self-described middle-class respondents. That compares with 16 percent who said the same of Democrats. The survey showed 37 percent said Democrats were more likely to favor their interests while 26 percent said that of Republicans.

Romney's Challenge

"Obama is perceived as more sympathetic to the middle class," said John Sides, who teaches politics at George Washington University in the nation's capital and is working on a book about the election. "The challenge for Romney is that he is seen as someone who's competent, but you want to be seen as someone who's in touch. There's enough of a gap there that it may be a liability for him."