The acrimonious debate in Washington over how to avoid the so-called fiscal cliff has cast a pall over shopper sentiment, retail experts say, as consumers head to the malls on the last Saturday before Christmas - typically one of the busiest shopping days of the year.

Talks to avoid the fiscal cliff stalled on Thursday when Republican lawmakers rejected House Speaker John Boehner's proposal aimed at winning concessions from President Barack Obama.

"The longer Congress delays making a decision on the fiscal cliff and the more uncertainty people feel, as we go toward Christmas, they would start pulling back on their spending," said Ron Friedman, retail practice leader at consulting firm Marcum LLP. "I don't think we're going to get a great pickup in the last few days here."

About 17 percent of the 1,514 Americans who participated in a Reuters/Ipsos poll conducted December 17-20 said the impending "fiscal cliff" was making them spend less this season.

U.S. consumer sentiment also plummeted in December as Americans were unnerved by ongoing negotiations, data showed.

The Thomson Reuters/University of Michigan's final reading on the overall index on consumer sentiment tumbled to 72.9 from 82.7 in November, worse than forecasts for 74.7. It was the lowest level since July.

"What could have been a merry Christmas is going to turn to a ho-hum Christmas, and we can thank our, you know, politicians for getting in the middle of it all," NPD analyst Marshal Cohen said. "It is like this great unknown puts a big damper on the consumer feeling confident to go out and spend more."

More than 60 percent of U.S. consumers have already finished more than three-quarters of their holiday shopping, according to a Reuters/Ipsos poll released on Thursday. This means retailers will have to offer deeper discounts to force Americans to open their wallets in the last lap of the holiday season.

The holiday quarter can account for about 30 percent of annual sales and half of profit for many chains, and experts including Cohen and Friedman see retailers pulling out all the stops this weekend and the week ahead to woo last-minute shoppers.

"The only way retailers now are going to be able to get a boost is by creating their own stimulus package, and that stimulus package is going to be markdowns," Cohen said.

Earlier this week, research firm ShopperTrak lowered its sales forecast for November and December and now expects sales to be up 2.5 percent, rather than up 3.3 percent.

Many retailers reported record traffic at the beginning of the season, but several, including Macy's Inc and Saks Inc, lost a lot of business because of Hurricane Sandy.

Earlier this week, Redbook Research said chain-store same-store sales rose 2.2 percent so far in December, suggesting shoppers are indeed cooling their heels. Sales for the November-December holiday season look set to rise 4.1 percent to $586.1 billion this year after a 5.6 percent increase in 2011, according to a National Retail Federation forecast.

"Retailers are going to be pretty challenged this year in trying to get beyond all this," Cohen said, referring to a string of events this holiday season that have weighed on U.S. shoppers including the hurricane, gridlock in Washington and a recent shooting at an elementary school in Connecticut.

NRF sees 2013 retail sales rising about 2 to 2.5 percent if the fiscal cliff is averted. If not, sales would be essentially flat for the year, the trade group estimated in a study with Macroeconomic Advisers.

(Editing by Matthew Lewis)

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