Fundamentals-only models rely on the theory of retrospective voting. That is, these models regard the election as a referendum on the incumbent government’s performance, in particular, its economic performance. The idea is that voters reward the incumbent party for good performance and punish it otherwise.
What differentiates fundamentals-only models from fundamentals-plus models is that the former are structural and do not use any public opinion variables (e.g., presidential approval), wheres the latter do.
The PollyVote’s fundamentals-only component forecast is calculated as the simple average of the available models in that category.