He had Indiana locked but was compelled by his personality to attack Cruz’s father as having something to do with JFK’s assassination.
Anecdote: I know two people who will not vote in November because this pushed them over the line. I think stupid Trump will hurt the down ticket.
Trump was reacting to Cruz’s father saying the election was about good (his son) vs. evil (Trump). I don’t have a problem with Trump responding to that and there could have been many ways for him to do that and look good. Stooping to the level of the National Enquirer wasn’t one of them.
One option for the Republican Party is to reject Trump and have him form a third party. This is a viable option if the polling indicates that Trump will lose to Clinton. The Republican Party is going to lose anyway so they might as well maintain integrity.