Final answer:
The main argument centers on the role of increasing violence in Congress and deepening sectional tensions over slavery as a direct lead-up to the Civil War, with slavery being the primary issue at stake.
Step-by-step explanation:
The author's main argument in the excerpt from Field of Blood: Violence in Congress and the Road to Civil War depicts the increasing sectional tensions and violent confrontations in Congress as key precursors to the Civil War, ultimately culminating in armed conflict over the issue of slavery. In particular, the discussions around the statehood of Kansas, the fate of slavery's expansion, and violent episodes such as the Sumner-Brooks affair are highlighted as central to the argument that ideological battles and moral commitments about slavery were at the forefront of the national psyche, reflecting the high stakes and fervent opinions held by lawmakers and citizens alike. This argument aligns with the broader scholarly consensus that slavery was not just a side issue but the core cause and catalyst for the Civil War, demonstrated by episodes of political violence and legislative battles.