Stock index futures were lower on Tuesday after a strong rally in the prior session as investors awaited the results of a key vote by Slovakia on expanding the euro zone rescue fund.

With all the other member states having ratified a pact to boost the size and powers of the European Financial Stability Facility bailout fund, all eyes turned to Slovakia.

That government was forced to turn to opposition parties to push through a deal in parliament after the ruling party said it would abstain from a vote on a deal intended to contain the Greek debt crisis.

The deputy leader of Slovakia's largest opposition party, Smer, said he was confident the country would ratify a deal to expand the rescue fund as soon as possible despite the expected failure of a vote later Tuesday.

Any more delays could unhinge markets already under pressure from signs the crisis was spilling beyond Greece's borders.

Potentially adding to the nervous climate, Jean-Claude Trichet, head of the European Central Bank, said the debt crisis has become systemic and risks to the economy were increasing rapidly with Europe's banks in the danger zone.

Basically, markets are looking at the outcome of Slovakia vote, the outcome of its vote, the extension of the EFSF and of course Trichet speaking before the European parliament, said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.

It's just a little bit normal to see the market soften after yesterday's strong rally, but all eyes are on Slovakia and I suspect a positive vote would unrattle premarket nerves and the rally probably will continue.

S&P 500 futures fell 4.1 points and were below 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 lost 13 points, and Nasdaq 100 futures shed 6 points.

Investor will shift focus to the start of the earnings season, with results due from top U.S. aluminum producer Alcoa Inc after the closing bell. Earnings season could offer insight on the extend of the damage from a global economic slowdown.

Global economic concerns have sparked a precipitous drop in metals prices in recent months. In the past week, analysts have lowered their consensus earnings estimate for Alcoa.

Sony Corp <6758.T> sees little cheer in the coming holiday shopping season amid global consumer gloom and has few options to cope with the euro's tumble against the yen, its chief financial officer said in an interview.

Car rental company Dollar Thrifty Automotive Group Inc said it had received no final acquisition proposals from Hertz Global Holdings Inc or other parties at the October 10 deadline and will continue with its stand-alone plan.

European shares fell 0.6 percent early Tuesday on the way to snapping a four-day rally ahead of the Slovak vote, and Asian shares rose after China moved to support its stock market by buying shares of major banks. <.EU>

U.S. stocks jumped 3 percent on Monday after a pledge by German and French leaders to come up with a plan to tackle the debt crisis, lifting the S&P 500 above its 50-day moving average for the first time since late July, a bullish technical signal.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)