Final answer:
The Republicans lost the 1856 election due to southern threats, lingering support for slavery in the North, and the division between Democrats and Know-Nothings. The correct answer is option A., B. and D.
Step-by-step explanation:
The Republicans lost the 1856 election in part because of the southern threats that a Republican victory would tantamount be a declaration of war. This fear among the southern states of potential conflict led to deep divisions and suspicion towards the Republican Party and their stance against the extension of slavery.
Additionally, the lingering support for slavery in the North and the strong divide between Democrats and Know-Nothings further contributed to the Republicans' loss. The North was not yet willing to let the South depart in peace, which increased tensions and ultimately played a role in the Republican defeat.
Overall, the 1856 election highlighted the deep-rooted differences over the fate of slavery and the growing sense of sectionalism that would eventually lead to the secession crisis and the Civil War.