Pessimism among U.S. consumers has been constraining businesses' ability to ramp up their hiring, increase their capital investments or restock their inventories -- a trend that has taken a toll on their second-quarter earnings.

Uncertainties revolving around the global economic slowdown, this year's presidential elections, and the effects of drastic tax increases and spending cuts that may roll out from Jan. 1 have pushed consumer demand downward. The shortfall has led to a decline in second-quarter revenues, prompting companies to trim their third-quarter and full-year earnings forecasts. DuPont and Co. (NYSE: DD), Texas Instruments Inc. (Nasdaq: TXN) and United Parcel Service, Inc. (NYSE: UPS) are the latest companies to do so.

United Parcel Service Inc.

Citing broad uncertainty, the Atlanta-based package delivery company trimmed its full-year earnings outlook after it missed analysts' second-quarter earnings estimates on Tuesday. UPS had earnings of $1.12 billion, or $1.15 a share - an increase of 2.2 percent from a year ago but 2 cents short of analysts' forecasts.

"Economies around the world are showing signs of weakening. Our customers are increasingly nervous," UPS Chief Executive Officer Scott Davis told analysts and investors on an earnings call. "Given these trends, we think current second-half economic forecasts for the U.S. are too high."

The company has revised its 2012 earnings forecast downward to a range of $4.50 to $4.70 a share, from its earlier estimate of $4.75 to $5, ruffling analysts, who had expected full-year earnings of $4.82.

Texas Instruments Inc.

Texas Instruments, which saw a 4 percent decline in its revenues from the year-earlier quarter, dimmed its third-quarter outlook on a drop in orders from its original equipment manufacturer customers and distributors. Its second-quarter revenues were $3.34 billion and its net profits were $446 million, or 38 cents a share. The earnings per share include 6 cents of charges arising from its restructuring efforts and its acquisition of National Semiconductor in September last year.

Though the company's analog revenues grew 13 percent from the year before, its wireless revenues fell 39 percent and its sales from embedded processing slipped 15 percent. The expiration of its supply agreements from factories it had previously acquired in Japan and China also took a toll on its revenues.

Analysts who follow TI are forecasting third-quarter revenues ranging between $3.21 billion and $3.47 billion. Their earnings per share projections range between 34 cents and 42 cents a share. TI forecasts are below the Street's expectations of $3.54 billion in revenues, or 50 cents a share. Acquisition and restructuring charges are expected to hit revenues further.

DuPont and Co.

DuPont was caught in the swirl of lower volumes and currency headwinds resulting from a rise in the value of the dollar. Its second-quarter profits dipped 3.2 percent to $1.18 billion, or $1.25 a share, from $1.22 billion, or $1.29 a share in the year-earlier period. Currency headwinds offset sales by 3 percent and volumes fell by 1 percent even as net sales climbed 7.2 percent to $11billion, missing expectations of $11.27 billion, or $1.46 a share, among analysts polled by Thomson Reuters.

DuPont executives told The Wall Street Journal that currency-related uncertainties could snip 12 cents to 14 cents a share from its third-quarter earnings. The foreseen impact for the full year is 30 cents to 35 cents -- 10 cents greater than estimated earlier.

The Delaware-based chemical manufacturer is now taking a grim view on its full-year adjusted earnings, which it expects will be on the lower end of its estimated spectrum of $4.20 to $4.40 per share. The company cited macroeconomic uncertainty, high tax rates and currency problems for its drop in profits and its reduced earnings outlook.

Overall weak earnings reports from large U.S. companies pushed the S&P 500 index downward. The S&P 500 dipped less than a point to 1,338 on Wednesday.