Trump not only won a plurality in the last six efforts, but a true (and in several places decisive) majority. Barring a sea-change in California, he is the presumptive GOP nominee. The only thing that could prevent his nomination now is for significant numbers of his delegates to not be certified at the convention. This could force a contested convention, but would be the kind of "smoke-filled room" games-playing that would seriously cheese off a number of voters. Like him or no, Trump will be the GOP standard bearer. It remains to be seen if there will still be a GOP when he is finished.

Clinton is just as likely to win the nomination as Trump, and need not fear a rules committee tossing her into the cold.

What will be decisive is whether one or more notable "3rd party" efforts are mounted -- none are likely to win, but they could siphon votes away from one of the major candidates in key states. Ralph Nader's Green Party run is what killed Gore in Florida in 2000 and got George W. into the oval office.