Complex sentence: The seat was a marginal to safe Labor seat from 1958 until the 1975 election which saw permanent large two-party swings away from Labor in a few rural seats − 13.5 percent in Chaffey, 15.5 percent in Mount Gambier and 16.4 percent in Millicent.
Split into simpler sentences: The seat was a marginal to safe Labor seat from 1958. The 1975 election saw permanent large two-party swings away from Labor in a few rural seats − 13.5 percent in Chaffey, 15.5 percent in Mount Gambier and 16.4 percent in Millicent.