NEW YORK (CNNMoney.com) - The National Retail Federation on Tuesday upped its holiday sales forecast to a 6 percent increase from an earlier 5 percent, saying that a fall in gas prices had given retailers more reason to be optimistic for a better season overall.

"As gasoline prices decrease, consumers are finding a little extra padding in their budgets," NRF chief economist Rosalind Wells said in a statement.

"Nearly every retail category has seen strong sales growth in the past few months, indicating that retailers will see positive gains as consumers continue to spend this holiday season," she added.

The NRF now expects total sales for the critical November-December holiday shopping period to reach $439.53 billion. But that's still below last year's 6.7 percent holiday sales growth.

The two-month period can account for as much as 50 percent of retailers' annual sales and profits.

According to the group, the revision marks the first time that the NRF has ever officially upgraded its forecast during the holiday season.

It previously forecast the 5 percent sales gain back in September.

"Recent consumer spending has surpassed our expectations, and we expect this momentum to continue through the holiday season," NRF president and CEO Tracy Mullin said in a statement. "Though our forecast remains cautious, we are confident that the holiday sales increase will be better than we originally anticipated."

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