Over the past two weeks, the GOP primary race has become a contest between Donald Trump and Ted Cruz, the two frontrunners in both national polls and in the state Iowa.

The pair will stand next to each other at tonight’s GOP primary debate in South Carolina, positioned center stage as a result of their standing in the polls. The contest between them is tonight’s main event.

The Trump/Cruz squabble is mind-numbingly stupid at almost every level. It is not a contest of ideas or policies, but a battle of attitude and insult, of culture-war positioning and social media put-downs. In other words, it reflects the awful current state of the GOP primary.

Trump kicked off the current round of hostilities by charging that Ted Cruz may not be eligible for the presidency, because he is not a “natural-born citizen.” Cruz, who was born in Canada, responded by releasing his mother’s birth certificate, making it clear that she was a U.S. citizen. There is no real question about whether Cruz is eligible—he is—but Trump’s attacks have worked anyway, distracting Cruz on the campaign trail and raising some doubts about his candidacy. In a Bloomberg Politics/Des Moines Register poll of Iowans, 15 percent indicated they felt “bothered” by the fact that Cruz was born in Canada. Conservative voters have become so feverishly opposed to immigration that even the hint of a foreign connection is enough to make them wary of a candidate.