King, who has been in Congress for a decade, said on Twitter late Friday the Senate race would be a distraction, The Des Moines Register reported.
"A Senate race takes me out of urgent battles in Congress that can't wait until 2015. Many thanks to all," he said.
Next year's Senate contest is the first open one in Iowa in four decades. Harkin defeated one-term Republican Sen. Roger Jepsen when he was first elected in 1984.
Rep. Bruce Braley, D-Iowa, is the only declared Democratic candidate for the seat. No Republicans have announced candidacies, apparently because they were waiting for King to make a decision, the newspaper said.