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," the Trump ally-turned-critic said.
The Republican had faced pressure to step aside and allow the party to unify around a viable challenger to Mr Trump.
He did not endorse anyone as he bowed out of the race.
But on a hot mic moments before he announced he was ending his bid, Mr Christie predicted that Nikki Haley, who has been gaining on Mr Trump in some polls, was "going to get smoked, and you and I both know it".
"She's not up to this," he added. He also said another rival, Florida Governor Ron DeSantis, was "petrified".
Mr Christie, 61, who nationally has been polling in the low single figures, said 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 the former president, who he accused of "putting himself before the people of this country".
"Donald Trump wants you to be angry every day because he is angry," he added.
His departure comes five days before the Iowa caucuses, 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 nominee in July, before going on to challenge the Democratic nominee - likely Joe Biden - in the November general election.
Mr Christie had been urged to drop out ahead of the second contest this month in New Hampshire, where some surveys indicate Ms Haley could be catching up with Mr Trump.
The north-eastern state has a large faction of unaffiliated voters and has been known to deliver unpredictable results. Mr Christie has been polling at 12% there and many of his supporters may now back Ms Haley.
In a statement, Ms Haley called Mr Christie "a friend for many years" and commended him "on a hard-fought campaign".
But her opponents were quick to chime in on social media.
Mr DeSantis said he agreed Ms Haley is "going to get smoked".
Mr Trump said he "might even get to like [Mr Christie] again" following his "very truthful statement" about Ms Haley.
The former president'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".
This was Mr Christie's second unsuccessful bid for the Republican nomination - he lost to Mr Trump in 2016.
Mr Christie had hoped to act as an attack dog for a field of Trump rivals who dared not cross the Republican base, which remains deeply loyal to the former president.
The strategy resulted in some zingers during the primary debates, but without Mr Trump on stage, Mr Christie failed to land any direct blows.
Matthew Bartlett, a Republican strategist based in New Hampshire, said Mr Christie had faced up to political reality.
"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.
