Trump calls Jeb 'an embarrassment to the Bush family'

Trump, by comparison, continues to dominate the field, seemingly becoming stronger with each new, inflammatory statement he makes, the latest being a call to ban Muslims from entering the country.

On the evening of the last Democratic presidential debate of 2015, Saturday Night Live instead looked back at the last GOP debate with all nine candidates represented - but with ex-Florida Gov. Jeb Bush and former-HP CEO Carly Fiorina on the receiving end of the most brutal mockery.

"It was so sad to watch him (at Tuesday's debate)", he said. Bush said, referencing Trump's garbled answer on modernising the nuclear triad intercontinental ballistic missiles, strategic bombers and submarine-launched ballistic missiles. "He has no clue".

Bush's stump speech covers the same topics it did two weeks ago, but the governor seems to more freely invoke criticism of Trump, much in the same way he has done with President Obama and Hillary Clinton, continuing to question Trump's seriousness as a candidate and even his intellectual curiosity.

On Thursday, Bush told CNN's John Berman that the approval Trump received from Vladimir Putin this week was a nod that he himself would not have wanted to receive. Bush is competing with Kasich, New Jersey Gov. Chris Christie and Florida Sen. In New Hampshire, Bush is not letting up on his jabs, partially casting his decision to go after Trump as a move to save the Republican Party.

You can not insult your way to the presidency. "A guy like that should not be the front-running candidate of our great party - that is not how we win".

The first ballot is in New Hampshire in early February.

Still, Bush has a long way to climb.

That, Bush said, is ultimately up to the voters. Just when it appeared he was done with his answer, Bush started his rant against his GOP rival.

Later Saturday, Trump continued his attacks on Bush during a campaign rally in Cedar Rapids, Iowa, calling him "incompetent" and "terrible".