Final answer:
Stephen A. Douglas opposed James Buchanan's support of the Lecompton Constitution because it violated the principle of popular sovereignty by impermissibly trying to establish a pro-slavery government in Kansas without proper consent from its residents.
Step-by-step explanation:
James Buchanan's support of the Lecompton Constitution infuriated Stephen A. Douglas because it sought to establish a government without the consent of the people, which betrayed the principle of popular sovereignty. The Lecompton Constitution was a controversial document that offered Kansans a choice between a constitution with slavery or one without slavery that still protected the rights of slaveholders, a deceptive choice that Douglas and many northerners viewed as a sham. Despite threats from Buchanan, Douglas knew that opposing the Lecompton Constitution was crucial for his political future, particularly as he faced re-election in Illinois and needed to align with anti-slavery sentiment.