U.S. online sales jumped 15 percent this holiday season, spurred by “aggressive” marketing by Web-based retailers, a research firm said.

Online sales were $36.4 billion from Oct. 31 through Dec. 22, compared with $31.5 billion a year earlier, said MasterCard Advisors’ SpendingPulse, a Purchase, New York-based firm that measures retail sales by all payment forms. Online apparel sales grew the fastest, or 26 percent, while electronics were up 12 percent and department stores 11 percent.

Daily online sales exceeded $1 billion on six days this season, which was twice as many days as a year ago, said Michael McNamara, a vice president at SpendingPulse. The growth meant online sales grabbed a larger share of the season’s total sales, he said.

“It’s a pretty positive story,” McNamara said in an interview. “Retailers started earlier, in the second week of November, and were more aggressive with their tactics — discounts and promotions. That seemed to drive growth.”

Amazon.com Inc., the largest online retailer, is likely to sell more than 8 million Kindle electronic-book readers this year, at least 60 percent more than analysts have predicted, according to two people who are aware of the company’s sales projections.

Tiffany & Co., the world’s second-largest luxury jewelry retailer, said Nov. 24 that the average size of orders on its Web site and through its catalogs increased in its latest three-month period, contributing to a 7 percent gain in sales in those channels.

Avoiding Crowds

At Macy’s Inc., the second-largest U.S. department store chain, online sales surged 32 percent in November. That’s more than five times faster than the 6.1 percent increase for the Cincinnati-based retailer’s sales that month at stores open at least a year, which include the revenue from Web transactions.

Some online shoppers may have been avoiding crowded stores on days like Black Friday, the day after Thanksgiving and one of the biggest shopping events of the year, McNamara said.

Nineteen percent of apparel sales happened online this season, compared with 17 percent last year, McNamara said.

Buying increased after consumer confidence climbed in December to the highest level in six months, helped by rising stock prices and a better jobs picture. At the same time, Saks Inc. and other chains limited promotions and discounts, forcing shoppers to pay full price for popular items.

Holiday sales may advance 3.3 percent, which would be the biggest gain since 2005, according to a forecast from the Washington-based National Retail Federation. The NRF had previously forecast an increase of 2.3 percent.

Sixty-two percent of adults said they expected to spend the same or more on holiday purchases this year than last, according to a survey from the NRF released Dec. 15.

Consumer spending accounts for about 70 percent of the U.S. economy, and the holidays generate about 30 percent of annual revenue for retailers.