Oct. 24 - U.S. economic growth eased slightly last month, according to a monthly index released Thursday. Americans are watching who will be the next president. The Chicago Fed's National Activity Index fell to -0.28 in September from -0.01 in August, the Fed said.A reading above zero indicates that economic growth is averaging positive. Meanwhile, the index's three-month moving average fell to -0.19 from -0.14, further signaling a generally weak trend in economic activity ahead of the uncertain presidential election in early November. Historically, the diffusion index of the Chicago Fed's National Activity Index has generally been above -0.35 during periods of economic expansion, while the figure has now risen to -0.16 from -0.21 in August.