From roughly Teddy Roosevelt’s era through Richard Nixon’s, [the Republicans] became slightly more centrist/liberal. Then starting in the late 1970s — about the time Nixon’s “Southern Strategy” was kicking in, and the South switched from being the Democrats’ stronghold to the Republicans’ — House Republicans became dramatically more conservative. Here’s the chart showing recent movements.
Graph source and article: http://www.theatlantic.com/politics/archive/2013/10/why-this-is-not-just-washington-dysfunction-in-1-more-graph/280161/