WASHINGTON — Small companies will probably remain a missing element of the current U.S. economic expansion as their role in driving growth continues to wane, according to economists at Citigroup.

Payrolls at firms with fewer than 500 employees accounted for less than 50 percent of the total work force for the first time in 2008 during the recession and have barely recovered, according to their research. After hovering close to 50 percent, small businesses’ share of gross domestic product began dropping in 2001 to reach about 45 percent in the latest available data.

In contrast to the gains in confidence for their larger counterparts, sentiment among smaller companies remains at recessionary levels, reflecting a confluence of obstacles, including globalization and a lack of credit as local banks consolidate. That means the world’s largest economy will lose support from the businesses that helped boost growth and productivity through innovation.

“Small businesses are going to be under pressure for quite some time to come,” said Nathan Sheets, the New York-based global head of international economics at Citigroup, who conducted the research. “Competing globally means establishing networks of distribution, communication, transportation, and it’s easier to do that as a large firm.”

After peaking at 55 percent in 1987, small-businesses’ share of the U.S. work force began declining in the 1990s. Citigroup’s analysis of Census Bureau data shows the slump was exacerbated by the 2007-2009 recession, the deepest and longest in the post-World War II era. The figures cover the period through 2010, the latest available.

Hiring by small companies still accounts for the lion’s share of gains in payrolls. Nonetheless, even that dominance is fading as the jobs they created accounted for 61 percent of the total in the 10 years through 2007, down from 65 percent in the decade ended in 1997 and from 77 percent in the similar period to 1987, according to Citigroup’s research.

The total number of small businesses is also falling, decreasing to 4.9 million in 2010 from a peak of 5.3 million three years earlier.

More recent figures from the National Federation of Independent Business indicate little has changed since 2010. The group’s small-business sentiment index in December hovered around an almost three-year low reached the previous month, according to a report last week.

“The environment for small businesses is not good,” said William Dunkelberg, NFIB’s chief economist and a professor at Temple University in Philadelphia. “Sales are weak, there’s regulatory bombardment, tax changes, all the uncertainty about the economy. People are thinking the odds are not as good as they might have been at other times, and so they’re not going to put their money on the table.”

A survey of 1,482 small businesses by the U.S. Chamber of Commerce confirmed the bleak outlook. More than half of respondents in the fourth quarter projected the economic climate for their firms will worsen over the next two years, according to a report last week. Less than 20 percent said they added employees last year, and the same share said they planned to hire in 2013.

Conversely, a survey of chief executive officers’ outlooks conducted by the Business Roundtable, which represents larger companies like Chevron and Cisco Systems, showed its index rose above its pre-recession peak in 2011 and continued to show expansion as of December, even as it cooled in the second half of last year.

To combat the small-business slump, Sheets, the Citigroup researcher, said policy should aim to spur entrepreneurship, which he says is the real source of job creation and invention.