Yes, the GOP has the chance to retain the majority in the United States Senate. Even if a great deal of voters seem to be in favor of Democrats, the fact that the GOP is in the majority in the Senate now shows they have always had a lot of support.
If Trump gets the nomination, no way the GOP keeps the Senate. Are people seriously not aware of the polls showing voters in the Senate battleground states think Obama's SCOTUS nominee should get a hearing like the Constitution says? And that the GOP Senators in those states are following The Turtle off the cliff in whining that they won't do it?
the GOP has begun losing favor among the majority of people because of their stance on many issues. As they hear about more deadly shootings, more and more people are beginning to support more gun control, something the GOP is against. If they do not change their views on the matter they will lose their majority.
The American people have seen the lack of leadership provided by the GOP. They have repeatedly created blocks against the progress that the president has tried to make, while refusing to find any sort of compromise. The American people are starting to realize that the GOP is a divisive party and they will not retain the Senate majority.