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Boulder, Colo. — Jeb Bush entered the 2016 sweepstakes last December full of bravado and subtle suggestions that his candidacy left no room for fellow Floridian Marco Rubio.

Nearly a year later, the former two-term governor and political giant in his state found himself chasing after the first-term U.S. senator, his one time protege, in a crucial presidential debate in a somewhat desperate bid to resuscitate his flagging campaign. Bush, 62, trails Rubio in most state and national polls, and his attack was the clearest signal yet that he now views the 44 year-old, not front-runner Donald Trump, as his chief rival.

"Everybody here understands the Marco Rubio's an outstanding performer. But we're running for the highest office in the land," Bush campaign manager Danny Diaz told reporters following the debate, broadcast Wednesday by financial news network CNBC. "The difference-maker is whether you have the record of accomplishment, the leadership skills, demonstrated for the biggest job in the country. I think there's a real question whether Sen. Rubio does."

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