Trump challenges Cruz over Canada birth

Republican frontrunner Donald Trump has attacked Ted Cruz over his birth in Canada, saying it raises questions about his presidential eligibility.

In the latest Republican debate for White House hopefuls, Trump told his rival: "There's a big question mark over your head." The constitution mandates the president be a "natural born citizen" of the US.

Issues of national security, the economy and foreign policy have also played heavily in the debate. The debate came just two weeks before the first real test of the campaign, when voters in Iowa pick their Republican and Democratic choices for president.

"Cruz acquitted himself well, cementing his status as the front-runner's chief opponent," writes Howard Kurtz for Fox News. "But Trump didn't suffer, and in fact may have had his strongest debate performance... The two-man top tier remains just that, way ahead of the rest of the field."

"For much of his career in Washington, Ted Cruz has been dismissed as a cartoonish sideshow," Michael Barbaro notes in the New York Times. However, he "did not just dominate much of the Republican debate, he slashed, he mocked, he charmed and he outmanoeuvred everybody else on stage".