The latest round of polls shows Donald Trump in as strong a position as he's ever been among national samples of Republicans, although Ted Cruz appears to have caught up to him in Iowa, home of the nation's first delegate selection event. One might expect that Trump, as the leading candidate, would therefore attract the bulk of attacks from rival candidates in Tuesday's debate—and that Trump might in turn direct fire at Cruz, who now represents a major threat to his chances in the Iowa caucus.
Instead, Trump sailed through the debate without becoming the target of sustained criticism from the rest of the field. Jeb Bush and Rand Paul took shots at Trump—Bush in particular cannot contain his open exasperation with Trump's antics, and clearly finds Trump's ascendance to be maddeningly incompatible with his own theory of the universe—but neither man has the popularity or credibility with the conservative base of the Republican Party to draw much blood, and Trump simply swatted them away. But Cruz, and to a lesser extent Marco Rubio, passed up several opportunities to attack Trump (Cruz was openly invited to do so by the moderators near the end of the debate), even though they are currently Trump's main rivals in the race. For his part, Trump returned the favor by disowning his own previous description of Cruz as a "maniac" when it was raised by a questioner.
Rubio and Cruz preferred to train their fire on each other. Cruz criticized Rubio on immigration (from the ideological right), while Rubio criticized Cruz on surveillance and military policy (ditto). As these policy stances represent each candidate's most notable departure from conservative doctrinal purity, the attacks were hardly a surprise, and these issues will probably continue to be raised as long as Cruz and Rubio remain in the race.
It seems likely that Rubio and Cruz subscribe to a similar strategic view of the nomination contest as it currently stands. If Trump is destined to fade, they reason, there is little advantage in attacking him now, since he will leave a large chunk of Republican voters up for grabs who will likely be reluctant to transfer their support to a candidate who had criticized their former hero. If Trump is not destined to fade, then both Rubio and Cruz want to be left standing as the primary non-Trump alternative in the race as the field narrows after Iowa and New Hampshire. Under either scenario, strategy dictates that they attack each other rather than the front-runner, who thus winds up getting something of an easy ride even as he continues to top the field.
At some point down the road, the strategies of Rubio and Cruz may diverge. The chief difference between their positions is that Iowa is probably a must-win state for Cruz, whereas Rubio only needs to finish respectably there. If Cruz is unable to pull away from Trump in Iowa, he may be forced to revoke his current non-aggression pact with Trump (and, if Cruz does start to pull away, Trump may abandon it himself). For now, however, we are left with a leader in the polls who is facing attacks not from the other top-tier candidates, but from those who are struggling to gain any traction whatsoever. As Trump remarked contemptuously on Tuesday in response to Jeb Bush's gibes, "I'm at 42 [percent], and you're at 3." A mean thing to say, perhaps—but not an inaccurate one.