Complex sentence: The Republican Party, dominant in the North, secured a plurality of the popular votes and a majority of the electoral votes, so Lincoln was elected the first Republican president, but before his inauguration, seven slave states with cotton-based economies formed the Confederacy.
Split into simpler sentences: The Republican Party, dominant in the North, secured a plurality of the popular votes and a majority of the electoral votes, so Lincoln was elected the first Republican president. But before his inauguration, seven slave states with cotton-based economies formed the Confederacy.