Complex sentence: Historians in recent decades argue that the conservative tradition has played a major role in American politics and culture since the American Revolution, However there has been an organized conservative movement playing a key role in politics only since the 1950s.
Split into simpler sentences: Historians in recent decades argue that the conservative tradition has played a major role in American politics and culture since the American Revolution. However they have stressed that an organized conservative movement has played a key role in politics only since the 1950s.