Trump has two problems with the Republican Party, the opposition of moderates like Romney and Ryan, and the opposition of conservatives like Ted Cruz and the editors of National Review. He can't win with both of these powerful factions against him, and he can't govern effectively if both factions in Congress work together to thwart him. Unfortunately, Republicans have seen what happens to Republicans who cross over and back Trump. Sarah Palin and Chris Christie are now totally delegitimized by their support of Trump, earning scorn and ridicule from all sides.
What that means is that even though Trump will get the nomination, it is still dangerous to one's reputation to support him. It is also dangerous to oppose him, since there is no doubt he will punish his enemies when he has the power. So I think most Republicans are going to see which way the wind is blowing before committing.
I've noticed that even Rush Limbaugh has not officially endorsed Trump, even though he is one of Trump's most strident cheerleaders, he refuses to make his support official.