At long last, Ted Cruz and Jeb Bush have mounted some really effective attacks on Donald Trump. Whether or not they have actually caused Trump's poll numbers to go down, they have prevented them from going up, which is all that's really needed to defeat Trump.
I've been pretty critical of the constant infighting between Rubio and Cruz, but I will say this about Ted Cruz: at least lately, he has stepped up to the plate in a big way against Donald Trump. He's running brutally effective ads against Trump, and got involved in several prominent dust-ups against Trump in the debate. Jeb Bush, likewise, has really found his stride, clearly winning nearly every exchange with Trump during Saturday's debate.
Now, granted, part of the reason for that has to be that Trump has placed both Bush (since day 1) and Cruz (since about January 1) in his crosshairs, so response is more or less mandatory. Rubio, he has mainly left alone for several months. On the other hand, Cruz was (rightly) criticized for months for leaving Trump alone just because Trump was leaving him alone, and this is no different.
Rubio is now doing what Cruz was doing in the beginning of the race: ignoring the threat of Trump, hoping to become the last non-Trump option and gambling that he will win a heads-up fight against Trump. That's a pretty good gamble, but it's still irresponsible for the future of the country.
Trump's upward momentum may have stalled, but it's foolhardy to assume that it can't resume. Trump is still a menace and in my view, everyone in the race has an obligation to treat him like the existential danger to the party that he is.
Moreover, one great way for Rubio to show that he learned something from his disastrous debate encounter with Christie would be to tangle with Trump directly. On Saturday night, Rubio again looked great without a bully on stage, but if the race really does boil down to him vs. Trump, conservatives want to know that he won't fold in the face of Trump's juvenile antics the way he did to Christie.
Cruz, at least, has proven that he can go toe to toe with Trump without flinching. It's time for Rubio to join the party and demonstrate that he can do the same.