Original sentence: The growing sentiment led to a dissatisfaction with the major parties—the Democrats were seen as too dependent on the votes of immigrants, and the Whigs were seen as ineffectual, and were largely in decline in any case.The so-called Know-Nothing movement was a American political movement of the 1850s.Thus activists began splitting off from the major parties and forming secret groups, coordinating their votes and throwing their weight en masse behind candidates sympathetic to their cause (regardless of political party).While significant in their own right, the concerns about the Pope largely exacerbated already-present  anti-Catholic feelings and feelings some claim were anti-immigrant held by many Americans
Edited sentence: The Know-Nothing movement was a American political movement of the 1850s.The growing anti-immigrant sentiment led to a dissatisfaction with the major parties—the Democrats were seen as too dependent on the votes of immigrants, and the Whigs were seen as ineffectual, and were largely in decline in any case.Thus anti-immigrant activists began splitting off from the major parties and forming secret groups, coordinating their votes and throwing their weight en masse behind candidates sympathetic to their cause (regardless of political party).While significant in their own right, the concerns about the Pope largely exacerbated already-present anti-immigrant and anti-Catholic feelings held by many Americans