Americans, who're increasingly optimistic about improving economic conditions, are expected to spend at a more rapid clip during the upcoming holiday shopping season than they did last year.

But that could change if the partial government shutdown that has forced about 800,000 federal workers off the job continues and causes shoppers to lose confidence in the economy.

The National Retail Federation, the nation's largest retail trade group, is expected to release Thursday a forecast that sales in November and December will rise 3.9 percent to $602.1 billion. That's above the 3.5 percent increase a year ago and the 10-year average in holiday sales growth of 3.3 percent.

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