Final answer:
Tensions in Texas escalated prior to the 1860 election because the Republican Party emerged as a major political force, focusing on halting the spread of slavery.
Step-by-step explanation:
Prior to the 1860 presidential election, tensions between unionists and pro-slavery Democrats in Texas escalated when d) The Republican Party emerged as a major political force. The Republican Party, formed in response to the Kansas-Nebraska Act, was firmly against the expansion of slavery. The election of Abraham Lincoln, a Republican, helped to drive the nation toward secession by southern states as they feared his presidency would threaten the institution of slavery. Events like the Fugitive Slave Act and the Dred Scott decision increased sectional hostilities throughout the 1850s, but it was the surge in power of the anti-slavery Republican Party that tipped the scales towards conflict.