Republican presidential contender Jeb Bush on Saturday rebutted Donald Trump's relentless mocking of him as a "low-energy" candidate, telling congregants of a Westhampton Beach synagogue of 16-hour campaign days crisscrossing the country.

"If you've been following the campaign, there's a candidate that says that some candidates are idiots and some candidates are this and some candidates are that," Bush said at the Hampton Synagogue. "I've been apparently the candidate that has low energy. So I'll just give you a little taste of the low-energy candidate's life this week."

Bush recited a long list of cities he's visited in the past six days, boasted of a "physical therapy workout" Friday with former Navy SEALs and said he would be jetting to North Carolina after his speech.

"The low-energy candidate this week has only been six days, 16 hours a day, campaigning with joy in my heart," the former Florida governor said.

Trump, the billionaire New York developer leading the crowded GOP field in the recent polls, has been ridiculing Bush as too low-key.

"I don't see how he's electable," Trump said in New Hampshire on Aug. 19. "Jeb Bush is a low-energy person. For him to get things done is hard."

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Bush, alluding to divisive rhetoric in the race for the White House, promised "to go campaign in every nook and cranny of this country with a hopeful, optimistic message. I'm not going to campaign to try to demonize my opponents. I'm not going to campaign in a way that tears this country apart even further."

The synagogue audience included Rep. Lee Zeldin (R-Shirley), who hasn't endorsed a presidential candidate. In an interview afterward, Zeldin said Bush looked more fired up.

"I personally saw more passion from him today than I ever have in the past," Zeldin said. He contrasted Bush's speech Saturday to his Aug. 6 appearance on Fox News, when "there were opportunities in the first debate for him and other candidates to shine more brightly by having a little more energy and more passion."

In his speech, Bush also attacked the Obama administration's nuclear deal with Iran as undermining the security of both the United States and Israel. Israel's government strongly opposes the pact. American Jewish opinion is divided.

"An unverifiable agreement that creates uncertainty -- and probably over the long haul will create nuclear proliferation in the region -- is not the right way to go," Bush told the crowd of hundreds who spilled from inside the synagogue onto two patios.

Bush lamented that while the Obama administration had improved relations with Iran, it had "ruptured" U.S. ties with Israel. He vowed to take a tougher line on U.S. adversaries.

"We will make sure that our enemies fear us a little bit," Bush said to loud applause.

Linda Pearlman, 65, of Patchogue, said after the speech that she is a registered Democrat but is so worried about the Iran deal that she will vote against any candidate who backs it. Democratic presidential front-runner Hillary Rodham Clinton supports the agreement.

"Iran is a country that cannot be trusted, but we're accepting their word, and that's not enough," Pearlman said.

Bruce Forman, 66, of East Quogue, said he liked Bush's vow for stronger U.S. leadership in the world.

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"That's how our respect in the world will come back. The current president has tarnished our respect," Forman said.

Bush was the third Republican presidential hopeful to speak to the modern Orthodox congregation this month. Sens. Marco Rubio of Florida and Lindsey Graham of South Carolina appeared on Aug. 15. Former Arkansas Gov. Mike Huckabee is scheduled for Sept. 4.