Complex sentence: Disappointed with Texas Republicans, McDonald increasingly exercised independence in supporting Presidential candidates: favoring Progressive Robert M. La Follette, Sr. and Democrats Al Smith and Franklin D. Roosevelt, before returning to the Republican Party to support Thomas E. Dewey in his presidential bid against Harry Truman.
Split into simpler sentences: Disappointed with Texas Republicans, McDonald increasingly exercised independence in supporting Presidential candidates: favoring Progressive Robert M. La Follette, Sr. and Democrats Al Smith and Franklin D. Roosevelt. He returned to the Republican Party to support Thomas E. Dewey in his presidential bid against Harry Truman.