BOULDER, Colo. — Jeb Bush and Marco Rubio fought for control of the Republican's establishment wing in Wednesday night's third GOP debate, as insurgent outsiders Donald Trump and Ben Carson defended the seriousness of their White House bids, underscoring the volatile two-track fight for the party's presidential nomination.Bush, once seen as the top Republican contender, entered the debate in the midst of the most difficult stretch of his White House campaign.