Donald Trump is losing support among reliably Republican demographic groups, which will undoubtedly impact Congressional races in November. Many of these supporters may stay home. He may also energize traditional Republicans and Democrats to come out, fearing a Trump presidency. Republican Congressional candidates cannot completely abandon Trump out of fear of losing his supporters.
It is possible that it could impact the Congressional race in the next election. First of all current members of congress who support Trump may find themselves losing support as well. Secondly it could cause people to change their minds about party loyalty and open up the race to more third party candidates.
Yes, if Trump discourages Republicans from voting for him, that is going to discourage those same Republicans from voting for other Republican candidates as well. In part, this is because some of them will be too unmotivated to vote at all. However, there are also those who will refuse to vote because of their association with the Republican candidate for the President of the United States.
Yes, I think that Donald Trump losing support from Republican demographic groups will impact congressional races because Donald Trump is the face of the Republican party at this point in time. His antics and words represent the Republican party hence if he loses support, congressional races will definitely be affected.