Trump leads in all national polls I have seen. If Kasich or Rubio drops out, Trump gets enough of their support to stay in front.
Moreover, Ohio is a winner-take-all primary. That 63 delegates. If Trump gets those, it is HUGE !!!
All but NC are winner-take-all states tomorrow. If Trump can take Ohio, Florida, Missouri and Illinois, that's around 280 delegates, and puts him very much on track to get over 50% of the delegates, regardless what happens to the other candidates.
Most of the remaining primaries are winner-take-all, so the numbers will add up quickly from now on. Trump needs under 800 to lock it up. He could get this before California.
IMHO, Trump's biggest obstacle is Kasich. If Kasich wins Ohio, then he could vault to the front for at least New York and California. That is 167 winner-take-all delegates, and that could be the one scenario that keeps Trump under 50% and therefore opens the door to a brokered convention. Trump winning Ohio reduces those odds dramatically.