Thom, when TR left the Republican party, he took a lot of the progressives in the party with him and formed the Bull Moose Party. After that party failed, the progressives couldn't get back into the Republican party. In other words, TR single-handedly changed the face of the modern Republican party, though not intentionally. When talking about the Republican party pre-TR and post-TR, you really need to make the differentiation.
Also the progressiveness of the Democratic party, came about more so because of FDR, than as an opposing platform to the Republicans. Though that was definitely part of it.