The GOP debate's three defining moments

The latest Republican presidential debate will be remembered for three exchanges. Which one voters find most important will determine Thursday night's winner in South Carolina.

The first was the "birther" argument between Donald Trump and Ted Cruz. Trump has been coyly implying Cruz isn't a natural-born citizen and therefore isn't constitutionally eligible to be president, or at least that the issue will wind up stuck in court. Cruz was ready to hit back.

"Back in September, my friend Donald said he had his lawyers look at this from every which way and there was no issue there. There was nothing to this birther issue," Cruz shot back. "Since September, the Constitution hasn't changed, but the poll numbers have. And I recognize that Donald is dismayed that his poll numbers are falling in Iowa, but the facts and the law here are really quite clear."

Not only did Cruz emphasize his own experience arguing before the Supreme Court and portray Trump as a flip-flopper on his signature birther issue. He even mentioned Trump's Scottish mother. But the Texas senator really hit Trump where it hurts: his poll numbers. And Trump didn't handle it well, invoking liberal constitutional lawyer Laurence Tribe on the legal issues and belittling Cruz's support in Iowa. On this back-and-forth, advantage Cruz.
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