Complex sentence: Thirteen Representatives, 8 Anti-Administration (Democratic-Republican) and 5 Pro-Administration (Federalist), had been elected in the previous election on an at-large basis, the last time that Pennsylvania elected all of its representatives at-large.
Split into simpler sentences: Thirteen Representatives, 8 Anti-Administration (Democratic-Republican) and 5 Pro-Administration (Federalist), had been elected in the previous election. That would prove to be the last time that Pennsylvania would elect all of its representatives at-large.