Complex sentence: Lincoln had worked hard to gain a reputation as a moderate in the party and hoped to be seen as a consensus second choice after Seward, who might be successful in those critical states, of which the Republicans had to win three to secure the election.
Split into simpler sentences: Lincoln had worked hard to gain a reputation as a moderate in the party and hoped to be seen as a consensus second choice after Seward. The Illinoisan wanted to be seen as a candidate who might be successful in those critical states, of which the Republicans had to win three to secure the election.