Let's think about this. What would HURT Lincoln's chances of winning? If Douglas supported Lincoln, his votes would go to Lincoln, HELPING him. (A is wrong). If the Constitutional Union supported Lincoln, THEIR votes would go to him and also help (B is wrong).
But at this point, the Democratic party was split over the issue of slavery, leading to the Southern Democratic party. If the parties were still united, they probably would have been able to overpower the Republicans. C is correct!
Answer: C. the democratic parties staying together rather than splitting.