Viewers of Tuesday night's Republican presidential debate could be forgiven if they thought they were watching two separate debates at once, not even counting the undercard. The primetime event was split into a competition between Marco Rubio and Ted Cruz on the one hand and Donald Trump versus almost everyone else on the other.

Those are also the dynamics of the GOP race as 2015 comes to a close and the Iowa caucuses beckon. You either believe that Trump's massive lead is real and as enduring as it has appeared to be since this summer, or you think — perhaps hope — that the reality TV star will yet implode.

If you think Trump is the genuine front-runner, as the billionaire himself clearly does, the strategy of struggling candidates like Jeb Bush or John Kasich makes sense. Either Trump must be taken down by someone presenting himself as the adult in the room or he will be the Republican nominee.

If you think Trump's lead is more illusory, the product of wall-to-wall media coverage and high name recognition but incapable of withstanding greater scrutiny or even a setback in Iowa, then the Cruz-Rubio contest makes more sense. Note that neither man went after Trump. They only attacked each other.

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