The feeling the pro-slavers (Democrats) had was that the anti-slavers (Republicans) were getting all the territory and they were losing numerical ballance politically. The slave to non-slave states ratio was getting further and further in favor of the anti-slave political factions. The growth in states was very one sided since Texas was added in 1845. With the Republicans in power there hold on the power of the nation was guaranteed to continue to grow. The Democrats were doomed to losing their control in Congress for decades to come, with or without a war.
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David Upton