Republican U.S. presidential candidates including (L-R) Dr. Ben Carson, businessman Donald Trump, former Florida Governor Jeb Bush and Wisconsin Governor Scott Walker pose for a group photo before the start of the second official Republican presidential candidates debate of the 2016 U.S. presidential campaign at the Ronald Reagan Presidential Library in Simi Valley, California, September 16, 2015. [Photo/Agencies]

SIMI VALLEY, Calif. - Jeb Bush, once a heavy favorite for the Republican nomination, 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 presidential election.

A Bush campaign aide said Jeb will focus on his "hopeful, optimistic vision" for how he would lead the country and that if an opportunity presents itself to contrast himself with Trump, he will take it. "He's not going to shy away on the Trump front," said the aide.

Reuters/Ipsos opinion polling has Trump leading among Republican voters with 32.2 percent. Retired neurosurgeon Ben Carson is second at 15.8 percent.

Bush, the younger brother of former President George W. Bush, is 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, due to start at 8 p.m. ET (0000 GMT Thursday).

Trump has mocked Bush as a "low-energy" candidate, 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.

A 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."