If Donald Trump ends up getting the Republican nomination, two primary factors will be to blame. The first will without a doubt be John Kasich's dumb ongoing vanity parade, which seems likely to hand 58 delegates to Trump in Arizona and some smaller amount in Utah by preventing Cruz from reaching 50%. The second will be Marco Rubio's fateful decision to stay in the race through Florida.
It's hard to be mad at Rubio for sticking it out through his home state, I guess, even though it was perfectly obvious by the time that Michigan rolled around that his campaign was done for. It's clear, however, that doing so provided a huge boon to Donald Trump. Not only did Rubio not win Florida (or come especially close), his continued presence in the race likely cost Cruz victories in Missouri and North Carolina.
The ripple effects of his decision to stay in the race are still being felt even today, as Arizona goes to vote in the third-largest winner-take-all primary in the Republican primary. The problem for Cruz, who is hoping to score an upset today, is that early voting has been going on in Arizona for over three weeks. Thus, many Rubio supporters have already cast their ballots for him before learning that he dropped out of the race. Thus, the force of any late movement there will be blunted by the fact that so many people will have already voted for a candidate who is no longer in the race.
In the end, will this be enough to matter? Who knows. What is clear is that Trump (who has the largest ego of anyone I have ever seen) has been helped along at every step of this race by the vanity of a long succession of candidates - beginning with Ben Carson, continuing to Jeb Bush, moving on to Marco Rubio, and now culminating with John Kasich. If he is the nominee, all will bear part of the blame.