* Some funds doubt political will for fiscal cliff deal
* Short positions increase ahead of Q4 reporting season
* U.S. shares on loan up 3 pct to $358 bln in wk to Jan. 4
* Stock lenders raise cost to borrow shares by 5 bps
LONDON, Jan 9 (Reuters) - How durable is the Wall Street bounce following last week's U.S. budget deal? Not very, some speculators believe.
Hedge funds are betting that a rally in U.S. stocks after a retreat from the "fiscal cliff" will reverse as doubts grow that politicians are ready to sacrifice party interests to keep the world's economic engine running, early data shows.
On the cusp of a Jan. 1 deadline, Republicans and Democrats agreed a moratorium on a package of tax hikes and budget cuts critics claimed would tip the United States back into recession.
The news triggered sharp gains in the S&P 500 index, which rose 2.5 percent to 1,462 points on Jan. 2. But the momentum is fading, leading some funds and analysts to predict a tough near-term outlook for U.S. equities.
"The recent rally is an opportunity to open promising short positions. Taxes are going up in some shape and form and spending will have to be reduced," said Athanasios Ladopoulos, chief investment officer of hedge fund firm Swiss Investment Managers. "Both feed into negative sentiment down the road."
Data measuring demand to borrow U.S. shares - a proxy for the level of short-selling, or bets on a share price fall - reflects expectations that markets will falter when the next bout of negotiations collides with talks to extend a $16.4 trillion national debt ceiling in February.
According to Sungard Astec Analytics, the aggregate value of U.S. shares on loan rose by 3 percent to $358 billion in the week to Jan. 4, as sceptical funds bet on falls in consumer confidence and company earnings.
That compares with a peak of $404 billion, seen in June when the Federal Reserve rowed back on employment predictions and cut 2012 economic growth forecasts to 2.4 percent from 2.9 percent.
By contrast, the aggregated value of shares in the FTSE 100 on loan fell 4 percent to $1.4 billion in the week to Jan. 4, while the equivalent for the STOXX Europe 600 dropped 3 percent to $6.6 billion.
Because such bets are struck privately, it's tough to pinpoint exactly when shares are expected to fall. Some bets may be pegged to the impending corporate earnings season, while others will be timed to exploit February's looming fiscal cliff worries.
SHORT, SHARP SHOCK
But even top stock market performers are seen suffering share price volatility until a compromise on cuts and taxes is reached.
Short-sellers are speculators who borrow shares then sell them in the hope of being able to buy them back at a cheaper price, before returning the stock to the original owner.
"While we are positive on U.S. equities for the year, the possibility of a short, sharp contraction on news flow is material in our view," equity strategists at BNP Paribas warned in a note, arguing equity valuations looked over-optimistic.
"The trailing price-to-earnings ratio of 15 times is above long-term averages and earnings growth has slowed to a crawl at best, compared with a consensus forecast for 10 percent," the note said.
Stock lenders - typically long-term investors such as pension funds who can earn a fee by loaning out a stock at little risk to themselves - ha ve also spotted an increased appetite to bet on falling stock prices and have raised the cost to borrow shares by 5 basis points (bps) to about 75 bps on aggregate over the first week of 2013, Astec data shows.
This brings borrowing rates closer to the 78 bps average earned on U.S. equity lending in 2012.
"There is much unfinished business ... not to mention the much bigger question about how the U.S. can meet its long term spending commitments in the face of an ageing population," Ian Kernohan, economist at Royal London Asset Management said.
"Given the polarised nature of U.S. politics at the moment, trying to sort all this out will be an uphill task."
The S&P 500, which rose 13.4 percent in 2012, closed 0.3 percent down at 1,457 on Tuesday. Some commentators say much of the recent growth in U.S. stocks is not due to an influx of optimistic buyers, but short-sellers closing out old bets from 2012 before embarking on a fresh set of short positions.
Heavily-shorted firms like $3.7 billion-valued U.S Steel Corp and $1.9 billion Advanced Micro Devices saw volumes of stock on loan drop by 15.1 percent to 17.5 percent and 18.6 percent to 14.6 percent re s pectively in the past month.
This data from financial information firm Markit supports the argument that bears, not bulls, are perversely largely responsible for driving the recent upward move in U.S. stocks.
Certainly a negative view is not mainstream for the year as a whole.
The consensus forecast from respondents to a Reuters poll in December was for the S&P 500 to finish 2013 close to its lifetime high of 1,576.09 set in October 2007.
But signs of fresh short-selling coupled with Friday's underwhelming December jobs data is putting pressure on market optimists.
Speaking at an investment forum hosted by asset manager Notz Stucki on Tuesday, Anatole Kaletsky, a financial economist and Reuters columnist, said cyclical factors such as weak housing markets have been major headwinds but there was evidence these have been neutralised.
But it may take time for this view to be adopted by many consumers. A Reuters/IPSOS online poll of U.S. consumers on Monday found four-fifths of respondents were bracing for another economic downturn.
Additional data from Markit showed sharp increases in demand to short a range of U.S. stocks who stand to lose from a dip in consumer sentiment.
But the list of the 30 most heavily-shorted U.S names in the week to Jan. 4 spans most sectors including pharmaceuticals, machinery and aerospace & defence, indicating broader pessimism in the market as well as cyclical or stock-specific concerns.
The volume of bets on a fall in the share price of $7.75 billion Tiffany & Co for instance shot up 17 percent last week to 6.4 percent, more than double the 3 percent average short interest on individual S&P stocks.
Demand to short leisure dot-com TripAdvisor Inc was up 11.6 percent to 5.2 percent and Dr Pepper Snapple Group Inc saw stock volumes on loan jump 15.1 percent to 7.8 percent.
"I am of the opinion that when Q4 earnings season starts investors will come to realise that the prospects for 2013 are not that bright," Ladopoulos said. "When the market turns down, it will take with it many of those too optimistic investors."