Final answer:
Kansas was the state that became the testing ground for popular sovereignty, particularly evidenced by the violent conflict known as 'Bleeding Kansas' in the mid-1850s.
Step-by-step explanation:
The state that became the testing ground for popular sovereignty was Kansas, especially during the mid-1850s conflict known as "Bleeding Kansas". After the Mexican-American War (1846-1848), the principle of popular sovereignty was considered for the territories acquired from Mexico. This principle was later applied to the Kansas-Nebraska territories in 1854, after the Kansas-Nebraska Act nullified the Missouri Compromise line. This act allowed the settlers of a territory to decide by voting whether or not slavery would be allowed, which led to significant conflict between pro- and antislavery factions in Kansas.
The situation escalated to violence, culminating in a period of civil unrest dubbed "Bleeding Kansas", as both sides attempted to influence the outcome of the popular sovereignty votes. This precursor to the American Civil War highlighted the dangers of using popular sovereignty to decide divisive issues such as slavery.