If you read through this politico article getting responses from campaign pros, on whether or not it's a lock for Trump, you'll notice that Bob Shrum is the only one who tells the reality of the situation. Bob Shrum, whom was taken to task in CTG, the longtime ago days, but he's right on in his take:

Donald Trump is not a lock now, of course—but he is irrepressible and maybe inevitable, despite the Politico Caucus’ tendency to convert wishes into analysis. Trump speaks to the alienation, anger and reaction of a wide swath of the base. No one will stop him now—except himself. But so far, as hard as he’s tried, he hasn’t trumped Trump. The GOP establishment will gather against him. But it appears poised to be crushed—before resigning itself to Trump. That will please Democrats. Jeb Bush is gone now, and Democrats are rooting for anybody but Marco Rubio. From a Democratic perspective, I would endorse Trump as a likely November loser, but my first choice for the GOP is actually Ted Cruz, an extremist more likely than Trump to lose, lose, lose.

Cruz is actually in fairly decent shape going forward, to win a number of the southern states, including Texas, on March 1st. Rubio is getting crushed in Florida. He and Kasich are the only electable Republicans, but they are also the least likely to win at this point. I'm sure it's still a possibility, of course, but realistically, it's just not there. I can be persuaded that Cruz might win. This sounds highly plausible:

On March 1, 12 states with a combined total of 588 delegates – nearly a third of the total — will get their turn. Delegate totals on that day are titled heavily toward the six Southern states, where Cruz and the outside groups supporting him are better organized than their rivals. That includes March 1’s biggest prize, Cruz’s home state of Texas, which awards 152 delegates and where the most recent polling shows him ahead of Trump.

...Kansas, Kentucky and Louisiana, three Cruz-friendly states with a combined 132 delegates to award, will hold their contests four days later on March 5, along with Maine.

Winner-take-all states do not begin voting until March 15, by which point more than 40 percent of delegates will have already been awarded. Both Ohio, with its 63 delegates and Florida with its 99 will vote that day – and the prospect of John Kasich hanging in long enough to capture his home state would only further muddle the delegate math.

Deeper into the map, major prizes like 95-delegate New York, which votes on April 19, and 172-delegate California, which votes on the last day of primary contests on June 7, will award delegates by congressional district.

There is not a path for Rubio, but there is one for Cruz?! Now imagine though, that Rubio stays in till the end, getting enough votes to get 2nd and 3rd place delagates, and Kasich stays in, and takes all of the Ohio WTA delegates. Cruz and Trump fight it on and on and on. We get to the GOP primary, and Trump & Cruz must become the ticket to have a majority.

Seriously, a fiasco operative could not make up a better scenario. Let it happen! Every 3-5 decades, one party or the other completely implodes with nominating a candidate that has no chance. To my chagrin, along with Hunter Thompson, that was McGovern in '72 ('84 was a weaker version imo). The GOP has not had an event like that since '64, and no one under 70 remembers that event; and those that do (calling Bob Dole) are listened to anymore. It's gone from the Republican voters mental collection of "what not to do to avoid a general election massacre."

It's almost gone from the Democratic mind as well, but it being 60 and above, and how baby-boomers still own the Democratic establishment, it's much more entrenched to avoid this scenario. Nothing like it was in the late 80's and the 90's, but still there, with the party elders and its voters, who are predominantly much older, and more younger. It was the latter group, which was strongly motivated for Sanders, that Clinton will have to figure out how to motivate soon. But there is plenty of time, and fear of Cruz/Trump will do the job.