Former New Jersey Governor Chris Christie has dropped out of the presidential race with a parting shot at frontrunner Donald Trump.

"I am going to make sure that in no way do I enable Donald Trump to ever be president of the United States again," he said.

The Republican had faced pressure to step aside and allow the party to unify around a viable challenger to Mr Trump.

This was Mr Christie's second unsuccessful White House campaign.

Mr Christie's departure comes days before the Iowa caucus, the first of the state-by-state contests in which Republican voters will pick their preferred candidate for president.

The eventual winner will be anointed the Republican Oval Office nominee in July, before going on to challenge the Democratic nominee - likely Joe Biden - this November.

Mr Christie, 61, announced that he was suspending his campaign at a town hall event in the US state of New Hampshire on Wednesday afternoon.

He devoted much of his remarks to a plea to Republican voters to reject Mr Trump.

Mr Christie accused the former president of "putting himself before the people of this country".

"Donald Trump wants you to be angry every day because he is angry," he added.

Mr Christie did not endorse any of the few other contenders still standing in the race.

But on a hot mic moments before he announced he was dropping out, Mr Christie predicted that another contender, Nikki Haley, was "going to get smoked, and you and I both know it".

"She's not up to this," he added. It comes a day after he publicly expressed doubts that Ms Haley could stand up to Mr Trump.

Mr Christie had faced pressure from within an anti-Trump faction of the Republican party to drop out, so as to allow someone like Ms Haley or Florida Governor Ron DeSantis to emerge from the pack.

Ms Haley, a former UN ambassador, has been presenting a possible threat to Mr Trump in New Hampshire.

The early-voting state has a large faction of unaffiliated voters, and has been known to deliver unpredictable results.

Reacting to Mr Christie's exit, Mr Trump's political action committee said in a statement that he had "already vanquished eight challengers before a single vote has been cast because Republican voters want a strong leader who will reboot our economy, secure our border, make America energy independent again, and keep our families safe".

Mr Christie had hoped to act as attack dog for a field of Trump rivals who dared not cross the Republican base, which remains deeply loyal to the former president. This strategy resulted in some zingers during the primary debates, but without Mr Trump on stage, Mr Christie failed to land any serious direct blows.

Matthew Bartlett, a Republican strategist based in New Hampshire, said Mr Christie eventually realised he could not win and wanted to "ensure that he is not playing the role of a spoiler".

On Wednesday, Mr Christie said his campaign was based on the conviction that "I would rather lose by telling the truth than lie in order to win."

But opinion poll after poll showed Republican primary voters did not want to hear Mr Christie's particular truth.

"Republican voters do not want to hear the same attacks that they've heard from Democrats or even the media for the better part of eight years," Mr Bartlett said.

"They just don't want to hear it. There's nothing you can tell a Republican voter about Donald Trump that they have not yet heard over the past decade."