WASHINGTON — The U.S. Commodity Futures Trading Commission this week voted to reject a request from the Securities Industry and Financial Markets Association and the International Swaps and Derivatives Association to delay implementation of position-limit rules aimed at preventing excessive speculation in commodity markets. The 3-2 vote to reject the request was the same margin by which the C.F.T.C. approved the plan in October. The two trade groups in part claimed the rules were not necessary and may actually harm the industry. The trade groups in December sought to block the new rules in a lawsuit against the C.F.T.C., which earlier this week asked a federal appeals court to dismiss the case. The C.F.T.C. plans to phase in the new position limits during 2012. The initial proposal to limit contract holdings stemmed from soaring commodity prices in 2008.