Republican presidential candidates Donald Trump and Ben Carson are threatening to boycott the next GOP debate over its proposed format, underscoring a rare political alliance between the leading outsider candidates. In a joint letter to CNBC's Washington bureau chief Thursday, the billionaire businessman and retired neurosurgeon told the hosting network they will not appear at the Oct. 28 debate unless it's capped at two hours with commercials and the candidates are allowed to speak directly to the camera at its opening and close.
