Final answer:
Slavery and anti-immigrant nativism eroded political party loyalty by causing internal disagreements and the fragmentation of existing parties like the Whigs. Nativism gave rise to the Know-Nothings, while the Republican Party attracted anti-slavery supporters by emphasizing the impact on free labor. This tumultuous political landscape shaped significant elections such as the presidential contest in 1856.
Step-by-step explanation:
Issues of slavery and anti-immigrant nativism significantly weakened the loyalty and cohesiveness of political parties in the mid-19th century United States by leading to internal divisions and the eventual dissolution of parties such as the Whigs. The Whig Party failed to unify around a strategy concerning slavery and, with their wait-and-see approach, they could not capitalize on the weakening of the Democrats due to the Kansas-Nebraska Act. Nativism, particularly anti-immigrant sentiments, contributed to the rise of the Know-Nothings, who were initially successful in drawing support away from other parties due to widespread nativist attitudes during a period of increased immigration.
The Republican Party began to attract anti-slavery support, particularly from those who were concerned about the expansion of slavery's impact on nonslaveholding whites. This focus on 'free labor' enabled the Republican Party to package antislavery sentiments without advocating for full racial equality. Elections during this period, such as the contest in 1856, reflected the divided nature of the American electorate on issues of slavery and immigration. Ultimately, the Democratic Party managed to project itself as a bulwark against disunion during these fractious times, with candidates like James Buchanan capitalizing on fears of divide.