Bush's name recognition and money '€” he recently hauled in $100 million-plus in fundraising '€” have kept him near the top of the Republican pack, where he has saved his most aggressive criticism for Democrat Hillary Rodham Clinton.

Bush was originally a front-runner in a crowded field of 17 Republican candidates seeking the party's nomination. Now slipping in the polls, he has abruptly changed course, following some Republican competitors who acknowledge that Trump, a billionaire businessman, has become a serious threat that must be dealt with head on.

"There's a big difference between Donald Trump and me," Bush declared in New Hampshire on Thursday. "I'm a proven conservative with a record. He isn't."

Bush has succeeded in raising far more money than his competitors, yet seven months after first signaling serious interest in a White House bid, the former Florida governor has yet to resonate with the vast majority of the Republican electorate. His polling numbers are stagnant, he faces continued questions about his family connections, and influential Republican activists remain skeptical of his conservative bona fides.

"I've never met a single grassroots voter who supports Jeb Bush," said Mark Meckler, a co-founder of the conservative anti-tax tea party movement.

If there is any solace for Bush's team, it's that he's not alone. Virtually every candidate not named Trump has suffered in recent weeks as the New York businessman caught fire with frustrated voters and sparked an anti-establishment backlash.

Trump isn't taking the pressure off his rivals.

"I don't see how he's electable," Trump said of Bush on Wednesday night in New Hampshire, later describing him as a "low-energy person" who has trouble getting things done.

Bush's team downplays any serious concern, noting that his poll numbers are "steady," he hasn't yet begun spending money on advertising, and that most voters aren't paying serious attention six months before the first votes are cast.

"Having a steady vote share in New Hampshire the summer before the primary is a good place to be," Bush spokesman Tim Miller said.

Bush may have created more problems for himself this week, however, while trying to capture some of the enthusiasm created by Trump's immigration rhetoric, including his calls to end birthright citizenship.

Bush said he believes there should be greater enforcement against pregnant mothers who cross the border to have children who then gain U.S. citizenship, referring to those children as "anchor babies." Facing a Democratic-fueled backlash, Bush defended his use of the term Thursday, but stressed that he believes people who are born in the country should have American citizenship.

Beyond his new battles with Trump, Bush has consistently faced questions from voters who are skeptical of putting a third Bush in the White House.

George W. Bush sent out a fundraising appeal on his younger brother's behalf. Asked if that conflicts with his characterization that he is his own man, Bush snapped back.

"Is that a contradiction?" he said. "I've got my own record. I've got my own life experience. I'm blessed to have a brother that loves me and wants to help me, over and out." (++++)

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