Caterpillar Inc. reported stronger-than-expected earnings on Thursday and raised its full-year forecast, but warned that a number of factors could weigh on its performance next year, and its shares fell as much as 2.8 percent.

The world’s largest maker of earth-moving equipment, which last year had its worst one-year decline in sales since the Great Depression, said it expected sales next year to snap back to pre-recession levels.

But the company told investors during a call after its earnings announcement that 2011 sales would be skewed toward lower-margin products, raising concern among investors that the incremental margin improvements recorded in recent quarters may be coming to an end. That would add to the challenges Caterpillar faces next year, which already include increased pension and retiree medical expenses, higher research and development expenses related to new emissions rules, and a higher tax rate.

“There’s a lot of risk to the bottom line,” said Morningstar analyst Adam Fleck.

“And there are a lot of uncertainties as far as developing market growth goes. We’ve already seen China try to stem their growth a little bit. Brazil’s raised interest rates. And you’ve got political concerns here in the U.S.,” Fleck said.

CALLS FOR U.S. “GROWTH AGENDA”

With less than two weeks to go before the Nov. 2 mid-term elections in the United States, Caterpillar’s earnings statement contained an uncharacteristically blunt political plea from the company’s management.

Chief Executive Doug Oberhelman, who took the helm at the end of the second quarter, said that while Caterpillar was expecting positive economic growth in the United States, “the recovery is weaker than we’ve seen historically, particularly given the depth of the 2009 recession.”

“To drive economic growth,” he said, “we encourage government policy makers to advance pro-business initiatives and a growth agenda. In addition, they should avoid policy decisions that may create trade tensions between the United States and other key trading partners and avoid tax policy that puts U.S. multinationals, like Caterpillar, at a competitive disadvantage.”

Caterpillar, which also makes diesel engines and gas turbines, reported a third-quarter profit of $792 million, or $1.22 a share, up 96 percent from $404 million, or 64 cents a share, a year earlier. Sales rose 53 percent to $11.13 billion.

Analysts, on average, had expected the Peoria, Illinois-based company to earn $1.09 share on sales of $10.65 billion, according to Thomson Reuters I/B/E/S.

Caterpillar said in its statement that continuing economic growth in the developing world has been key to improving sales.

Developing countries, including China, India and Indonesia, accounted for about half the growth in machinery sales volume during the quarter, as increased infrastructure construction fueled demand for building equipment and higher commodity prices lifted demand for mining machines, Caterpillar said.

“In addition, sales in developed countries have improved substantially after deep declines in 2009. While demand has increased, dealer new machine inventories and rental fleets have remained relatively flat, and the age of rental fleets has not improved, and that should be positive for us as we move forward,” Oberhelman said in the statement.

Production of mining and related support equipment increased more than 25 percent from the second quarter to the third, and the company said previously announced investments in new plants and equipment would allow it to continue ramping up production next year.

As a percentage of sales, the company’s operating profit nearly tripled to 10.7 percent of sales from 3.8 percent a year ago.

But that kind of margin improvement may be more difficult next year. During the investors’ call, Caterpillar executives repeatedly stressed the headwinds the company expects to face in 2011.

Mike DeWalt, head of investor relations, said, “We still have a pretty weak recovery going in the developed world. It is improving, but those percentage increases are off pretty low levels.”

Caterpillar shares reached $81.18 on Thursday before the call, but then slid to $77.51. In late afternoon trading, the shares were down 1.8 percent at $78.43 on the New York Stock Exchange. The shares closed Wednesday at $80.32.

“Things are getting better,” said Eli Lustgarten, an analyst at Longbow Research. “But guess what? It wasn’t the blowout that we saw for Eaton and Parker.”