Final answer:
President James Buchanan's support of the Lecompton Constitution, which would admit Kansas as a slave state, resulted in a deep division of the Democratic Party. The correct answer is option a.
Step-by-step explanation:
President James Buchanan's decision on Kansas's Lecompton Constitution played a critical role in deepening the national divide over slavery and significantly impacted the political landscape. Buchanan faced pressure from his proslavery advisers and decided to support the Lecompton Constitution, rejecting the desire of the majority in Kansas and effectively asking Congress to admit Kansas as a slave state based on the Dred Scott decision. This decision ultimately hopelessly divided the Democratic Party, as many northern Democrats, including Stephen Douglas, stood against it to maintain their political stance on popular sovereignty and their future political careers. The controversy and the political maneuvers around the Lecompton Constitution, combined with subsequent events such as the Lincoln-Douglas debates and John Brown's raid on Harper's Ferry, contributed to the national crisis leading up to secession and the Civil War.