James Buchanan won the Democratic nomination for the presidency in 1856 primarily because he had extensive administrative and foreign policy experience. Buchanan had served as Secretary of State, Minister to Great Britain, and had been involved in various diplomatic missions. This experience made him a highly respected and qualified candidate within the Democratic Party.
While the other options listed could have contributed to Buchanan's overall appeal, they were not the main reasons for his nomination.
Campaigning against the policy of popular sovereignty, which allowed territories to decide the issue of slavery for themselves, was not a key factor in Buchanan's nomination. In fact, Buchanan was known to support the idea of popular sovereignty, which aligned with the Democratic Party's position at the time.
Gaining fame as a western explorer and soldier did not play a significant role in Buchanan's nomination. While he did have some military experience, it was his administrative and foreign policy expertise that made him stand out.
Controlling the key swing state of Pennsylvania may have been a favorable factor for Buchanan, but it was not the primary reason for his nomination. His extensive experience and qualifications were more influential in securing his party's nomination.
Distancing himself and the Democrats from the Kansas-Nebraska Act, which allowed territories to decide the issue of slavery through popular sovereignty, was not a driving factor in Buchanan's nomination. In fact, Buchanan supported the Kansas-Nebraska Act and its principles, which aligned with the Democratic Party's platform at the time.
In summary, while the other options listed may have played a role in Buchanan's nomination, it was primarily his extensive administrative and foreign policy experience that led to his selection as the Democratic candidate for the presidency in 1856.