Complex sentence: Many people credited Kirkwood's speech and subsequent work with the success of the Republican Party in Iowa; that year he was elected to the Iowa Senate serving from 1856 to 1859.
Split into simpler sentences: Many people credited Kirkwood's speech and subsequent work with the success of the Republican Party in Iowa; that year he was elected to the state legislature. Kirkwood was elected to the Iowa Senate from 1856 to 1859.