In plurality, "top of the heap" voting systems, the winner need not secure a majority of the vote. In fact, not a single Republican presidential primary this year has been won with more than half the votes cast — even in Ted Cruz's big win in Wisconsin on April 5. Our plurality system is built only to handle a choice between two candidates, not three or more.
In recent days, both Donald Trump and Ted Cruz have suggested John Kasich should leave the Republican race because he's getting votes that otherwise would go to them. But as FairVote has regularly pointed out this cycle, we don't need to guess — pollsters instead simply need to ask better questions. Public Policy Polling regularly has done so, and last week in its national poll asked Republican voters' preference among Trump, Cruz and Kasich and then between just two of them. Trump's plurality lead of 42% to 32% for Cruz and 22% for Kasich shrinks to just 46% to 44% over Cruz once Kasich is dropped. That indicates how close the race would be nationally with ranked choice voting — and how, with ranked choice voting, no one would have to point fingers at another candidate for continuing to run. (As an aside, Trump and Cruz each defeat Kasich by more than 10% even though Kasich regularly polls best against Democratic candidates.)