Early signs of a slowing economy have investors nervous today. The ISM Index, a closely watched indicator of manufacturing activity, came in at 49.5% today, lower than the 51.7% investors expected. This indicates a slight slowdown in manufacturing activity and has pushed the Dow Jones Industrial Average (DJINDICES:^DJI) down 0.4% and the S&P 500 (SNPINDEX:^GSPC) down 0.5% as of 3:20 p.m. EST.

Given weak manufacturing data, it should be no surprise that General Electric (NYSE:GE), DuPont (NYSE:DD), and 3M (NYSE:MMM) are three of the biggest losers on the Dow today. Not only are these companies manufacturers, but they also rely on other businesses for much of their revenue. A slowdown in manufacturing on a large scale will have a big impact on these companies.

Energy stocks are also down despite the fact that oil is up slightly today. ExxonMobil (NYSE:XOM) and Chevron (NYSE:CVX) are down 0.6% and 1%, respectively, as investors worry about an economic slowdown. These companies are already dealing with declining consumption in the U.S., and further upward pricing pressure could hurt them going forward.

Only a few stocks on the Dow were moving higher today, including Cisco (NASDAQ:CSCO) and Travelers (NYSE:TRV), but neither was driven by material news.

This is one of the first real indicators that businesses and consumers are starting to cut back as we head toward the fiscal cliff. But as with the debt ceiling debate, the cutbacks may be lifted when uncertainty is cleared up. That means it's time to prepare ourselves by buying when uncertainty is high.

Fool contributor Travis Hoium has no positions in the stocks mentioned above. You can follow Travis on Twitter at @FlushDrawFool, check out his personal stock holdings, or follow his CAPS picks at TMFFlushDraw

The Motley Fool owns shares of General Electric Company and ExxonMobil. Motley Fool newsletter services recommend Chevron and 3M Company. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.