• by Mark Walsh  @markfwal, January 3, 2013

U.S. online holiday sales rose 14% to $42.3 billion from a year ago, according to new data from comScore, falling short of the firm’s projection of 16% e-commerce growth for the November-December period.

After starting the season strong with sales up 16% during Black Friday and Cyber Monday, that rate slowed failed to top 12% in the first three weeks of December because of concerns about the economic implications of the fiscal cliff, according to comScore.

“With Congress deadlocked throughout December, growth rates softened even further and never quite made up enough ground to reach our original expectation,” said comScore Chairman Gian Fulgoni. “While it is typical to see growth rates subside slightly during the week after Thanksgiving, the amplified and sustained lull this year came as something of a surprise.”

Cyber Monday (Nov. 26) wound up as the heaviest shopping day, with $1.46 billion in spending, followed by Tuesday, Dec. 4 with $1.36 billion, and Green Monday (Dec. 10) at $1.27 billion. The 2012 holiday season saw 12 days with more than $1 billion spending, beating last year’s total of 10 individual days.