Most observers expect a short-term compromise to be found that blunts the full effect of the tax increases and spending reductions, but economists say the risks to future growth are mounting. If a deal is not reached, the economy will grow at an annual rate of just 1.1 percent in 2013, according to a new forecast issued Thursday by Macroeconomic Advisers, with unemployment rising to 8.5 percent by the end of the year.

Unemployment now stands at 7.9 percent, still far above the level before the financial crisis and the recession, and it is not expected to come down significantly unless economic growth accelerates. For now at least, that seems unlikely.

“Over all, it was a disappointing report,” said Michelle Meyer, senior United States economist at Bank of America Merrill Lynch. The accumulation of inventories went from subtracting 0.1 percentage points from the initial estimate to adding 0.8 percentage points, she said.

“A lot of that inventory build was unintentional, which suggests a downside risk for the fourth quarter,” she said. “Businesses had expected stronger sales and consumer spending and were caught off guard.” Ms. Meyer said she expected the economy to grow by 1 percent in the fourth quarter of 2012.

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The government also reported on Thursday that first-time unemployment claims dropped by 23,000, to 393,000, last week. But Ms. Meyer cautioned that these figures were much more volatile than usual because of the Thanksgiving holiday and Hurricane Sandy.

The weak underlying data in the report on third-quarter performance prompted Maury Harris, chief United States economist at UBS, to cut his projected estimate of fourth-quarter growth to 1 percent from 1.6 percent. “Part of this is Sandy and part of this is the inventories,” he said.