Buchanan correctly believed that the Dred Scott decision would not expand slavery anywhere, but he promptly rejected an ideal opportunity to prove this to his northern critics. Kansas already had a large antislavery majority and was ready to become a free state whenever a free and fair election could be achieved. Instead, a convention at Lecompton, Kansas, elected by a small fraction of the eligible voters, wrote a proslavery state constitution and ruled that Kansas could vote for it with or without slavery but could not vote against the constitution itself. Only a handful voted in the December 1857 referendum, and the result was an overwhelming victory for slavery. With incontrovertible evidence that the Lecompton Constitution was acceptable only to a small minority of Kansans, Buchanan ignored the pleas of his own appointed Kansas governor and asked Congress to admit Kansas as a slave state. Kansas, he announced, was "as much a slave state as Georgia or South Carolina."
To southerners, Kansas was a slave state won in a fair contest. To northerners, the administration and its Kansas allies had violated the sacred democratic precept that the people of any state should be allowed to accept or reject any constitution in a fair election. In the Senate, Douglas increased Buchanan's hatred and damaged permanently his own presidential chances in the South by opposing the Lecompton Constitution as a fraud. Nonetheless, the administration and the southern leadership mustered enough votes in the Senate to approve the admission of Kansas under the Lecompton Constitution by a vote of thirty-three to twenty-five. In the House, a long and bitter debate punctuated by a free-for-all fistfight on the floor ended in a compromise that ultimately returned the constitution to Kansas for another vote. On 2 August 1858 the people of Kansas rejected the Lecompton Constitution by a six-to-one margin. Southerners were furious over the loss, while northerners were no less angry over what had been attempted. The northern vision of a slavocracy dominating the White House, Senate, and Supreme Court aroused defiance everywhere.
Buchanan responded to Douglas' apostasy on Kansas by using every power at his command against the senator's reelection in 1858. In Illinois, civil servants and newspapers dependent on federal contracts were ordered to oppose Douglas, and an administration Democrat became a third candidate. The contest featured the Lincoln-Douglas debates, which overnight made Abraham Lincoln a national figure but weakened Douglas still further in the South. Lincoln announced a formula on slavery that inspired moral comfort without imposing any disturbing sense of obligation. Slavery, he said, should be left alone in the South, but its containment should be firmly established to put it on the road to "ultimate extinction." This, he argued, would put the northern mind at ease and stop the antislavery agitation, which in turn would stop the southern demands and threats of secession. Douglas in his so-called Freeport Doctrine sought to reconcile popular sovereignty with the Dred Scott decision. Even though the Court had forbidden government action against territorial slavery, he said, a territorial legislature could effectively bar slavery by refusing to pass laws for its protection. Douglas was reelected, but President Buchanan had played a major role in promoting Lincoln's future and weakening the hopes of Douglas for the presidency.
However I have to mention this, as it's basic. I was surprised that your first sentence "Buchanan correctly believed Dred Scott decision would not spread slavery anywhere"
Actually it was precisely -- and only -- the Dred Scott decision that Jefferson Davis explained changed everything, because by the precise language the government had an obligation to protect slavery, and Davis used that in context of the DUTY to spread slavery into Kansas and beyond regardless, in spite of the will and vote of the public. I would be deeply honored if you would explain this, because it seems to me (And to Jeff Davis own explanation in his book "Rise and Fall" ) that Dred changed everything. By "protect" slavery, by the way, Davis meant protect it the way it was protected in the South -where it was a crime to preach, or speak publicly
, or even own books that were anti slavery.