The hope was that Rubio would rise and Cruz and Trump would stumble. That didn't happen. The defining development of the sixth Republican presidential primary debate was this: For the first time since the campaign entered its late stage, Ted Cruz was willing to skirmish with Donald Trump, and he did it more effectively than any of the dozen-plus other candidates who have tried before him. He also made an uncharacteristic error. Halfway through the evening, the Texas senator reprised a campaign trail attack on Trump's "New York values," knowing that Trump would retaliate by characterizing it as a slander...