donald trump is not on the ballot in november, not directly or indirectly, and his ability to act as commander in chief, which is his job, is not reliant on his control of congress the way it is in a westminister system. 

in the united states, these are entirely separate branches of government that have no relevance to each other.

only members of congress are on the ballot this november, and it is not the case that americans will be voting for the president by proxy. americans will be voting for their representatives, based on their own performance. that is a well understood feature of the american system that is very different from the british system.