Final answer:
The trend in Southern state partisanship in US house and presidential elections is the increase in voters aligning with national party platforms, resulting in a decrease in split-ticket voting and an increase in polarized, party-specific voting patterns.
Step-by-step explanation:
The specific trend identified in Southern state partisanship in US house and presidential elections is the increasing alignment of voters with national party platforms and the corresponding decrease in split-ticket voting. This means that voters who choose a certain party in presidential elections are now more likely to also support that party in House and Senate elections.
A clear example of this is the South beginning to vote overwhelmingly Republican in presidential elections during the 1960s and 1970s, whereas Democrats were commonly elected to the House and Senate. However, this changed in the 1980s and 1990s as the number of Democratic representatives began to decrease, indicating a stronger alignment with the Republican Party at the national level.
In terms of state legislature control, as of early February 2021, thirty states had Republican majorities in both the state house and senate. In contrast, eighteen states had Democratic majorities, and only in Minnesota was control split between the two parties. This shift towards partisan alignment at the state level further supports the national trend toward polarized, party-specific voting patterns.