From Jonathan Bernstein at Bloomberg:
Could John Kasich be single-handedly destroying a political party?
Either by helping Donald Trump win the Republican nomination or by dragging the party into an ugly contested convention, Kasich seems to be causing a lot of trouble.
Serious presidential candidates normally leave the race when they no longer have a chance to win. This is an important part of how the nomination system functions. If losers drop out, then voters in subsequent states — voters who might not follow politics enough to know which candidates are serious contenders and which aren’t — won’t waste their voters on the also-rans.