Complex sentence: The Republicans emphasized the danger of letting Democrats run the nation so soon after southern Democrats provoked the Civil War and, to a lesser extent, the danger a Democratic administration would pose to the recently won civil rights of southern blacks.
Split into simpler sentences: The Republicans emphasized the danger of letting Democrats run the nation so soon after the Civil War, which they claimed had been provoked by southern Democrats provoked. To a lesser extent, they also campaigned onthe danger a Democratic administration would pose to the recently won civil rights of southern blacks.