Roughly 550 donors from the Koch network met at a private retreat this weekend. They have reason to be happy given the state of the economy and a recently passed tax reform bill, but Donald Trump's erraticness is providing a unique challenge in their bid to maintain Republican dominance on Capitol Hill.

A three-way feud among the Republicans' leading White House contenders has escalated, with Republican Ted Cruz daring Donald Trump to sue him and dismissing Marco Rubio's charges of dishonesty just days before South Carolina's high-stakes primary.

An explosive feud between Donald Trump and Fox News Channel is overshadowing the final sprint to Iowa's presidential caucuses, injecting a new sense of chaos into the 2016 Republican contest.