Not quite market related, but I have seen somewhere that any incumbent house-controlling party (currently republicans) has lost the midterms in 34 cases out of 37 since the independence.
In terms of the impact, democrats, being a shadow cabinet (if this term even works for the US), will supposedly act as populists (anti-big companies agenda seems to be prevalent in the US) and block as much as they can when it comes to Trump’s initiatives. Interestingly, his administration has only recently repealed Obama’s ban on JPM’s growth even within the US. This bank could not expand its branches to other states (it is only represented in 20) for years and now plans to establish branches in additional 23 since the restrictions are lifted.
Similar moves are common for smaller corporations too but we are likely unaware due to their insignificance. Tax reform has also given big lump sums for the internationally operating businesses who now have motivation to repatriate their cash and boost their growth further.
The problem with democrats is that they are pro-regulations and pro-restrictions for businesses. If they win midterms, it is prudent to presume the market will react negatively and the corrections will continue.
But this is only my personal assumption.