At this point there is effectively no way for any of the other candidates to overtake Trump. If the Republican "establishment" really wants to stop him, their only course is to continue to fund Cruz, Rubio, and Kasich, keeping them in the race for as long as possible and denying Trump the outright majority (1,237 delegates) he needs to clinch the nomination. Then they can go into an open or "brokered" convention and hammer it out. In such a scenario the Republican power brokers are likely to do anything and everything — including committing political suicide — to deny Trump the nomination. Mitt Romney is often mentioned as a fallback candidate, perhaps with Marco Rubio as his running mate. But regardless of who they finally anoint, one thing is very clear: It will not be Donald Trump.
Trump, of course, will be magnificently outraged, claim to have been robbed (and he wouldn't be wrong), and leave to run as an independent. This, of course, will effectively hand the election to the Democrat, presumably the Lady Clinton. But Trump won't care. From that moment on his purpose will be to destroy the Republican candidate, whoever it is. Mormon God help Romney if it's him. I don't know why they think Mitt Romney is their guy to stand up to Donald Trump; he's just Jeb Bush with better bone structure. He's a weenie, and he won't be able to stand up to Trump. I wouldn't be surprised if the final results had Trump coming in second and the rump Republican candidate third.
This scenario is not only plausible, at this point it actually looks fairly likely. And a part of me thinks that it might be what Donald Trump wanted all along.