Republican Speaker of the House John Boehner, in a brief press conference Wednesday, said his chamber would pass a proposal that spares many wealthy Americans from tax hikes needed to balance the budget. Obama has threatened to veto the plan if it passes, while some Republicans oppose any deal featuring tax increases.

"This is enough to make people think talks have regressed, which is making people nervous," said James Dailey, portfolio manager of TEAM Asset Strategy Fund in Harrisburg, Pennsylvania. "We don't know how much of this is just political posturing, and we're probably past the point of automatic faith a deal will be cut."

Equities have been volatile this week, though they notched strong gains Monday and Tuesday as offers from Obama and Boehner indicated progress was being made in budget discussions.

That rally ended Wednesday as shares slumped, though they remain near a two-month high, as a rise in tensions in the talks threatened to unravel recent progress.

While many investors still expect a deal to avert the "fiscal cliff," steep tax hikes and spending cuts due to take effect in 2013 that could tip the economy into recession, agreement may not come as quickly as they had hoped.

S&P 500 futures rose 1.2 point and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures added 9 point and Nasdaq 100 futures rose 5.75 points.

Stocks have rallied recently on signs of progress in the negotiations, led by banking and energy shares, which tend to outperform in times of economic expansion. On signs of complications, however, many have turned to hedging their bets through options and exchange-traded funds.

Investors await the latest economic data for trading impetus; weekly jobless claims and the final estimate on third-quarter gross domestic product are both due at 8:30 a.m. ET (1330 GMT). The pace of growth is likely to be revised up to 2.8 percent from 2.7 percent, while claims are expected to rise to 357,000 in the latest week, from 343,000.

November existing home sales are due at 10:00 a.m. and are seen rising 1.3 percent, compared with 2.1 percent in October, while the Philadelphia Federal Reserve Bank's December business activity survey is forecast to come in at -3.0 compared to -10.7 in November.

"People are starting to feel vulnerable on the fiscal cliff, so any negative data could have an outsized impact on trading," Dailey said.

IntercontinentalExchange Inc (ICE.N) is in talks to buy NYSE Euronext (NYX.N), the operator of the New York Stock Exchange, according to a source familiar with the situation on Wednesday, in a multibillion dollar deal.

Google Inc (GOOG.O) agreed to sell set-top TV box maker Motorola Home to Arris Group Inc (ARRS.O) for $2.35 billion in cash and stock. Arris rose 3.2 percent to $15 in premarket trading.