Initially stung when Trump, 69, pinned the "low-energy" label on him, Jeb Bush is taking a more aggressive tack at the upcoming Republican debate on Wednesday.

"If someone comes at me, bam! I'll come back at 'em," he said at a weekend campaign stop, smacking a fist into a palm. 

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Bush has mocked Trump, branding him a Democrat in disguise with past positions favoring higher taxes and past expressions of admiration of Hillary Clinton, the front-runner for the Democratic presidential nomination.

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If a livelier Bush shows up at the second debate, it will be an adjustment welcomed by many of his supporters, some of whom have privately expressed concern that the former Florida governor, a favorite of the Republican establishment, could be in trouble after a dismal August.

 "They're mindful that everybody knows that he is George Bush's son and George W. Bush's brother but that people don't really know him," said a donor in touch with the Bush team. Aides say he aims to address a longstanding concern that voters see him as a possible third Bush president, not as a veteran politician with his own identity.

A Reuters/Ipsos opinion poll on Friday had Trump as the breakaway leader with 26.9 percent. Retired neurosurgeon Ben Carson was at 10.4 percent. Bush was at 7.3 percent. The poll had a credibility interval of 3.8 percent.