To provide evidence supporting that ethnic conflict among whites rivaled sectional conflict as a major political issue in the 1850s, some relevant information could include:

1. Rise of anti-immigrant/nativist political movements and parties, like the Know-Nothing/American Party, which opposed immigration and aimed to limit political influence of Catholic immigrants.
2. Passage of laws or proposed legislation aimed at restricting immigration or rights of immigrants, especially at the state level.
3. Outbreaks of ethnic violence or riots targeting immigrant communities, particularly those involving Protestant natives and Catholic immigrants.
4. Political debates, campaigns, or rhetoric by politicians emphasizing ethnic/religious tensions and the perceived threat of certain immigrant groups.
5. Voting patterns or election results showing significant anti-immigrant sentiment among native-born white voters in certain regions.

Specific examples like the harsh anti-Catholic rhetoric of the Know-Nothings, the Philadelphia Nativist Riots of 1844, or the political rise of figures exploiting nativism would help illustrate how ethnic divisions created major political conflicts and shaped the national discourse alongside sectional issues in that era. Contemporary accounts expressing concern over mass immigration’s societal impact could also substantiate the argument.