If you still harbored any doubts about the growing division in the Republican Party, the recent Republican Town Hall meetings (which involved all three Republican presidential candidates individually) should settle those doubts.
Last September, all the Republican candidates pledged to support the party's eventual nominee (and to not run an independent campaign against that nominee). All of that is out the window now. In the Town Hall interviews, all three Republican candidates refused to say they would support the party's eventual nominee.
This is not a surprise that Trump wouldn't support the nominee (and might even run against that nominee as an independent). When Trump made his pledge, it came with a condition -- that he believe he had been treated fairly by the Republican Party's leaders. Recently, those party leaders have initiated a "Stop Trump" movement, with plans being made to try and snatch the nomination away from him in an open (brokered) convention. It's easy to see why he would feel he's not being treated fairly.
It conceivable that Trump will launch an independent bid for the White House if denied the convention's nomination. That would be disastrous for the GOP, since a recent Rasmussen Poll showed that 24% of Republicans say they would be very likely to vote for Trump if he did. Even if he didn't run as an independent, those voters might vote for a third party (feeling they were cheated), or even worse just stay home on election day -- which would hurt the down-ballot Republican candidates.
It could be just as bad if Trump does get the nomination. There are a lot of Republicans who say they simply could not vote for Trump, and a party with Trump heading the ticket could also hurt down-ballot Republicans. And both Cruz and Kasich refused to say they would support Trump as the nominee of their party. If they can't get on board, why should other Republicans?
I don't think it's an overstatement to say the Republicans have a real electoral mess brewing. The party is seriously divided, and I'm not at all sure that division can be healed by November.