Gloves off for top tier in GOP race after debate

While the full field of Republican presidential candidates resumed campaigning Friday, it was Sens. Ted Cruz and Marco Rubio who emerged from last night's Fox Business debate the apparent main challengers to front-runner Donald Trump -- though New Jersey Gov. Chris Christie's feisty exchanges may have left some thinking what was once a 17-candidate scramble, now is a four-man showdown.

Or, maybe five, as former Florida Gov. Jeb Bush, who managed to get in a few shots at Trump, picked up the endorsement Friday morning of former candidate Sen. Lindsey Graham.

Trump, though, quickly dismissed the backing, tweeting that Bush’s “chances of winning are zero,” the same as Graham’s polling.

The exchanges all marked a newly aggressive campaign heading into the final stretch before Iowa.