Final answer:
The rejected proslavery constitution of Kansas was called the Lecompton Constitution, defeated by Kansas voters in a landslide vote in 1858 amidst heated national controversy.
Step-by-step explanation:
The proslavery constitution of Kansas that was ultimately rejected was called the Lecompton Constitution. Despite the initial support from President Buchanan and the Democratic-controlled Congress, the proposed constitution was highly controversial and divisive. In January 1858, Kansas voters defeated the Lecompton Constitution with a decisive 10,226 to 138 vote. The document was part of a larger struggle between proslavery and antislavery forces during the period of 'Bleeding Kansas', and its rejection was seen as a significant setback for the proslavery agenda. The Lecompton Constitution was designed to enshrine slavery in the new Kansas-Nebraska territories and faced considerable opposition, including that from Democratic Senator Stephen A. Douglas. Ultimately, the pressure and the presence of fraudulent activities, such as the acts of the 'border ruffians', underscored the contentious nature of this historical event.