Republican Representative Adam Kinzinger's refusal to endorse Trump hurts the candidate in the election. His own party is abandoning him, which is unprecedented. This will carry weight with the general electorate, which is not as ideologically driven as the core Republican voter in the primary. Kinzinger's refusal to endorse will also snowball and others will follow suit.
Donald Trump has, himself, managed to hurt his chances in the general election. By not having the endorsement of many leading Republicans will only hurt him more and his numbers will continue to fall in the national polls leading up to the November election. This may be the beginning of a trend among members of the Republican party wishing to distance themselves from Trump's rhetoric as he continues to dig himself into a deeper hole of which he will have not way of exiting.
Mr. Trump has had great success with voters largely based on his position as an "outsider." He has continually gone against traditional campaign methods and routinely upset the established Republican leadership, yet he seems to have managed to gain popularity and support in spite of this. One more Republican failing to endorse him is not likely to hurt him at this point.
Trump's supporters like the way that he upsets establishment figures. They admire his boldness in saying things that other people won't dare to say, and in expressing his outrageous views. Anyone who shows disapproval of Trump's behaviour is unlikley to have any impact on his support as it just proves to his supporters that he is on the right track.