Let me begin this by reiterating that I think we are perhaps past the point of avoiding a Donald Trump nomination absent some singular act of Divine Grace. But, if it is possible to stop Donald Trump, the way that happens is by Rubio getting out of the race, now, and campaigning hard for Cruz. Even then it will be difficult but that is the only way forward if "stop Trump" is actually a concern of either Ted Cruz or Marco Rubio (I do not believe that is the case. I believe both men are well aware that Reagan didn't quit and endorse Ford in 1976 because the presidency was the goal, not "stop Jimmy Carter.")
The first data point that says Cruz is the stronger candidate is the current state of polling. I posted on that earlier today.
The second data point is "second choices", that is, the putative preferred alternative if a voter's favorite candidate drops out:
— Sean Davis (@seanmdav) February 23, 2016
If Rubio leaves the race, 46% of his voters say they will vote for Ted Cruz. This brings Cruz up to ~30%+ of the electorate. Enough to compete with Trump and to probably beat him because Cruz is a much better and more disciplined candidate than Trump. On the other hand, 43% of Cruz supporters say they will vote Trump. Personally, I think you can throw the 17% of Carson voters into that group. This puts Trump at ~50% of the electorate. Game over.
This won't happen. Yesterday, I posted on how Rubio has said he will not attack Trump. That appears to be his strategy:
Rubio has yet to aggressively engage Trump — and those briefed on his strategy say he’s unlikely to do so on Thursday night.
The Florida senator has concluded that going after Trump would accomplish little because the businessman’s supporters are deeply committed and unlikely to swing Rubio’s way. Inciting a confrontation with Trump onstage would create drama but wouldn’t help the senator gain voters, something he badly needs as he looks for his first primary win.
Instead, Rubio’s team has decided his best bet is to focus fire on Cruz. They think the Texas senator’s voters are less locked in and could swing Rubio’s way should Cruz fade. The only way to dislodge Trump, Rubio’s advisers say, is to turn it into a two-man race — meaning that they first need to get Cruz out of the way.
Of course, if Trump chooses to engage Rubio — something he hasn’t yet done in a debate — Rubio will have to respond. And while Rubio may not directly take on Trump, he may do so implicitly.
Rubio will also be playing to the more mainstream, establishment Republican donors and supporters, arguing as he has shown in recent days that he is the only candidate who can unite the party and win the general election.
So Rubio is playing the role of the dog in the manger. He can't possibly win the nomination. His scurrilous and dishonest personal attacks on Cruz, attacks that in more polite era would have resulted in Rubio being horsewhipped through the streets, are not unifying the party. They are convincing non-Rubio voters that Rubio is incredibly dishonest and not worthy of their support.