Stocks fell on Tuesday morning, giving back some of Monday's gains as investors awaited the first presidential debate. On the economic front, the Commerce Department said that the U.S. trade deficit in goods rose 3.5% to $82.9 billion in August, reflecting strong imports of consumer goods and slower growth in exports. The increased deficit could weigh on 3Q GDP. Meanwhile, the Conference Board reported that its consumer confidence index rose to 101.8 in September, up from 86.3 in August and well above t