By Eric Morath 

WASHINGTON--Constrained American consumers are threatening to weigh on U.S. growth just after the economy regained its footing following a pullback early in the year.

Hamstrung by weak income gains, consumers are struggling to step up spending. That is providing perhaps the biggest challenge to a second-half economic breakout that already is at risk due to global unrest and persistent slack in the labor market.

Household spending declined a seasonally adjusted 0.1% in July from a month earlier, the Commerce Department said Friday. It was the first time spending fell in a month since January. Meanwhile, personal income, reflecting income from wages, investment, and government aid, rose 0.2% in July--the smallest monthly increase this year.

"On the income side, we're still lagging a bit, so consumers are having a hard time getting out of second gear," said Jim Baird, chief investment officer at Plante Moran Financial Advisors.

Consumer spending is the lifeblood of the U.S. economy, accounting for more than two-thirds of economic output.

Outlays slowed sharply to start the year during an unusually cold winter in much of the country. That contributed to gross domestic product contracting at a 2.1% annual pace during the first quarter. Household spending and the broader economy bounced back in the second quarter when GDP advanced at a 4.2% pace, the Commerce Department reported Thursday.

The latest numbers suggest growth could settle closer to a pace that is little better than the 2% gains averaged since the recovery began five years ago. Forecasting firm Macroeconomic Advisers on Friday cut its projection for third-quarter growth to a 2.6% annual pace from 3.1%. Barclays lowered its forecast by half a percentage point to a 2.2% pace.

Nashville-based home décor retailer Kirkland's Inc. has recorded sales growth so far this year, but Chief Executive Robert Alderson remains concerned about inconsistent shopping habits in what he called a "no or slow growth economy."

"We would still prefer a better jobs environment and a housing market to feel more comfortable with the state of the consumer," he told analysts last week. "Sustained economic optimism and predictable growth are required to produce both."

July's spending pullback was broad-based. Outlays fell on both durable goods, such as cars and appliances, and everyday items, including gasoline and groceries. Services spending was flat, though that could reflect a cooler-than-normal summer that kept utility bills in check.

On the surface, the weak spending figures appear at odds with accelerating job creation. The last six months saw the strongest stretch of payroll gains since 2006. Underpinning those gains, however, was hiring in low-wage fields such as restaurants, retailers and temporary jobs. At the same time, a historically high number of Americans aren't participating in the labor force or are working part time but would prefer a full-time job.

The feelings of consumers have improved of late. Consumer sentiment increased in August from July, according to a separate Reuters/University of Michigan gauge released Friday. That gain was driven by higher-income Americans. Households with incomes less than $75,000 are feeling more pessimistic.

"Higher wages have been slow to appear and gains in the stock market are not enjoyed by all," said Chris Christopher, an IHS Global Insight economist. "More widespread income gains are needed to get all consumers back on solid footing."

From a year earlier, incomes grew 4.3% in July without adjusting for inflation, a modest acceleration from the start of the year. Still, income gains remain below the annual average increase of about 6% seen during the past 35 years.

"I don't know that the confidence level is there yet for workers to leave a job for a higher salary or ask for a raise," said Navy Federal Credit Union economist Alan MacEachin.

The unemployment rate, at 6.2% in July, is down more than a percentage point from a year earlier. The rate might need to fall a half-percentage point further to put much upward pressure on wages, Mr. MacEachin said.

Write to Eric Morath at eric.morath@wsj.com