George W. Bush pitches in for Jeb as 2016 heats up

Jeb Bush might be his own man, but he showed Thursday he won't hesitate to enlist his brother George, the former president, in his own bid to win the White House in 2016.

George W. Bush wrote a fundraising email in support of his younger brother, who is locked in a tough race for the Republican nomination and is trailing bombastic frontrunner Donald Trump in poll after poll.

"I rarely send emails like this, but I wanted to make sure I asked for your support on behalf of my brother, Jeb," George W. Bush wrote in the first campaign message of its kind by the 43rd president.

"This is a consequential time in our nation's history, and we need a strong leader," he added.

"Jeb took on tough challenges as Florida's governor and delivered results. I know he will do the same as president."

George W. Bush remains deeply divisive in the United States, where many argue that his Iraq invasion was among the worst foreign policy blunders in US history.

While Jeb has insisted that "I am my own man," he has struggled to emerge from his brother's shadow. He was criticized by presidential rivals recently for enlisting several advisers who worked for George W. Bush.

- Trump lashes out -

The email hit inboxes as a new Quinnipiac University poll showed Jeb trailing "The Donald," as he is nicknamed, in three key swing states: Ohio, Pennsylvania and Bush's home turf of Florida.

It landed just as Bush began showing backbone against relentless rhetoric by Trump, who on Wednesday hammered Bush for his Iraq and immigration policies and ridiculed his deflated poll numbers.

"He's going down like a rock," Trump told a raucous, overflowing crowd in New Hampshire, which holds the first-in-the-nation primary.

"I don't see how he's electable," Trump taunted, as Bush spoke just miles away at his own New Hampshire town hall.

"You know what's happening to Jeb's crowd, right down the street? They're sleeping!" Trump boomed.

Bush returned fire from his stage, saying "Mr Trump doesn't have a proven conservative record" and "was a Democrat longer in the last decade than he was a Republican."

He continued the pushback Thursday, blasting Trump's "tidal wave of accusations or bombastic talk" on immigration.

Trump has unveiled an immigration plan that includes building a wall on the US-Mexico border, and rounding up and deporting millions of undocumented immigrants.

"This whole immigration debate is hurtful for a lot of people, really hurtful," Bush told reporters Thursday.

"There's a lot of people that share the immigrant experience, and what they hear is 'You don't think I'm part of this, part of this country.'"

Bush bristled when asked whether it was appropriate to insist he is his own man, yet enlist brother George's help in his campaign.

"Is that a contradiction?" Bush responded, visibly agitated.

"I have my own record, my own life experience," he added. "I'm blessed to have a brother that loves me and wants to help me. Over and out."