Final answer:
Stephen A. Douglas opposed the fraudulent proslavery Lecompton Constitution, worked with Republicans to defeat it, and broke with President Buchanan to advocate for popular sovereignty and to reflect the anti-slavery sentiment of his Illinois constituents.
Step-by-step explanation:
Stephen A. Douglas responded to the proslavery Lecompton Constitution of 1857 by opposing President Buchanan and advocating against the constitution's legitimacy.
Many northern Democrats, including Douglas, opposed the idea of admitting Kansas as a slave state under the Lecompton Constitution because it did not represent the will of the Kansas settlers.
Despite President Buchanan's support for the proslavery constitution and his warning to Douglas that opposing the administration would result in political ruin, Douglas stood firm on his principle of popular sovereignty, which held that the settlers of a territory should decide the issue of slavery.
In an effort to uphold this principle and secure his political future, Douglas worked with Republicans to defeat the Lecompton Constitution. As a result of separate elections held by proslavery and antislavery forces in Kansas, it became clear that Free Soilers—who opposed spreading slavery into territories—held the majority.
Furthermore, Douglas broke with Buchanan publicly, which led to a sectional divide within the Democratic Party. This divide became more pronounced as Douglas faced reelection in Illinois and needed to distance himself from proslavery interests.
Ultimately, Douglas denounced the Lecompton Constitution and its fraudulent process, showing that his commitment to popular sovereignty and political survival outweighed party loyalty.