Investors worried that Slovakia might not approve a plan to strengthen Europe's bailout fund. All 17 countries that use the euro must agree on the plan, which is considered essential to resolving the region's debt crisis. Sixteen countries have approved it so far.
Concerns about the debt problems have weighed on the stock markets for months. Investors worry that if Greece defaults on its debts, banks that own Greek bonds in Europe and in the U.S. would suffer. That, in turn, could slow the global economy.
About 45 minutes before the opening bell Tuesday, Dow industrial average futures are down 70 points, or 0.6 percent, at 11,301.
Standard & Poor's 500 futures are down 9, or 0.8 percent, at 1,182. Nasdaq 100 futures are down 10, or 0.5 percent, at 2,267.
The Dow rose 330 points Monday, its largest gain since Aug. 11. The S&P 500 rose 3.4 percent. Investors were optimistic after French and Germany leaders said they would finalize a "comprehensive response" to the debt crisis by the end of the month, including a plan to make sure European banks have adequate capital.
But, on Tuesday, fears about Europe returned. A "no" vote from Slovakia could rattle the financial markets and complicate European efforts to deal with the debt crisis that has already caused three countries to get bailouts.
In the U.S., aluminum maker Alcoa Inc. will become the first major company to report third-quarter results after the closing bell. Analysts expect earnings from S&P 500 companies to rise about 12 percent from the same period last year, according to data provider FactSet. Revenue is expected to go up about 11 percent.
In corporate news, Sprint Nextel Corp.'s stock fell 1 percent ahead of the opening bell. The stock has plunged 26 percent since Friday, when Sprint said that it wants to speed up plans to revamp its wireless network. Analysts say that will raise its expenses dramatically.