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ORANGE CITY, Iowa — Marco Rubio on Friday shrugged off Jeb Bush and the struggling former Florida governor's attacks on his surging candidacy, saying he was focused on his own campaign and wouldn't respond in kind.

The Florida senator served as the speaker of his state's house of representatives when Bush was governor, and they used to be political allies. But Bush, the one-time Republican 2016 front-runner, now trails Rubio nationally as well as in Iowa and other states. Rubio said he assumes that Bush's campaign advisors "convinced" the former governor that attacking him would help him get back into the contention.

Rubio remarked that he doubted the strategy would work, and warned that it was counterproductive for Republicans to undercut each other's fitness for the presidency if the goal is defeating presumptive Democratic nominee Hillary Clinton next November. Rubio spoke to reporters in the conservative, northwest corner of Iowa after addressing a gymnasium filled with about 1,000 likely GOP caucus goers, The crowd gathered to hear from him, New Jersey Gov. Chris Christie; businesswoman Carly Fiorina and 2012 caucus winner Rick Santorum.

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