By Steve Holland
SIMI VALLEY, Calif. (Reuters) - Once considered a heavy favorite for the Republican nomination, Jeb Bush will try to energize his presidential campaign on Wednesday when he goes into a prime-time debate where Donald Trump looms large.
The former Florida governor, an establishment Republican, has been overshadowed by Trump's bomb-throwing rhetoric in the contest to become the party's candidate for the November 2016 election.
Bush has to show some fervor in the debate in California, said Austin Barbour, senior advisor to a Super PAC that supported former Republican candidate Rick Perry.
"Look, Governor Bush has a fantastic, successful record in Florida. He has everything it takes to be a successful president. But I think he needs to go out there and show voters, Republican voters, that he’s got the passion for the job,” Barbour said.
Reuters/Ipsos opinion polling has Trump leading among Republican voters with 32.2 percent. Retired neurosurgeon Ben Carson is at 15.8 percent.
Bush, the younger brother of former President George W. Bush, is back in third place on 9 percent as his campaign struggles to take off.
Eleven candidates will be on stage for the debate at the Ronald Reagan Presidential Library in Simi Valley which starts at 8 p.m. Eastern (0000 GMT Thursday).
Trump has mocked Bush, 62, as being "low-energy" but the well-funded former governor has begun to hit back and has vowed not to be passive at the debate.
"If someone comes at me, bam!, I'll come back at 'em," he said at a campaign event in New Hampshire last week.
The main Super PAC supporting Bush launched an ad on its YouTube channel this week accusing real estate magnate Trump of being "in a very dark place."
Aides to Wisconsin Governor Scott Walker, another conservative favorite who is now trailing Trump, have promised that their candidate will also step up at the debate.
Walker is at seventh place in the Reuters/Ipsos poll and a Gallup survey shows that he is unfamiliar to 44 percent of Republicans even though he was often mentioned by pundits only a few months ago as a serious contender.
"Walker’s decline really kind of shocks me,” said Alan Abramowitz, professor of political science at Emory University. “I thought he was well positioned to at least make it to the final three or four.”
Trump, who dominated the first main Republican debate in Ohio last month, has dismissed possible attacks from rivals this time around.
"I hear they are going after me. Whatever. Whatever," he said at a campaign rally in Dallas on Monday.
The face-off, staged next to the retired Air Force One aircraft used by Reagan, is being hosted by CNN.
It will be the first time Trump has been on stage with former Hewlett-Packard CEO Carly Fiorina. Trump last week was quoted as saying of her: "Look at that face. Would anyone vote for that?"
A CBS News/New York Times poll said 63 percent of Republican voters had not yet decided who to support.
A poor showing on Bush’s part could just be another blip on the election timeline that might not prove fatal, said Steven Brams, professor of politics at New York University. “The debate tonight will not be do or die,” he said.
Also debating will be Ben Carson, Ted Cruz, Marco Rubio, Mike Huckabee, Rand Paul, John Kasich and Chris Christie.
Rick Santorum, Bobby Jindal, George Pataki and Lindsey Graham will participate in an 'undercard' debate at 6 p.m. Eastern (2200 GMT).
For more on the 2016 presidential race, see the Reuters blog, “Tales from the Trail”. (http://blogs.reuters.com/talesfromthetrail/)
(Writing by Alistair Bell; Additional reporting by Alana Wise and Michelle Conlin; Editing by James Dalgleish)