Outspoken Donald Trump-critic Senator Lindsey Graham drops his US presidential bid

US Senator Lindsey Graham speaking at a 'Stop Iran Rally' in New York in September.
US Senator Lindsey Graham speaking at a 'Stop Iran Rally' in New York in September.PHOTO: EPA

WASHINGTON (AFP) - Lindsey Graham, the hawkish Republican senator who wants thousands of US troops in the Middle East and warned Americans against nominating Donald Trump for president, announced Monday he is exiting the White House race.

"I am suspending my campaign, but never my commitment to achieving security through strength for the American people," Graham said in a video statement.

"The centrepiece of my campaign has been securing our nation. I got into this race to put forward a plan to win a war we cannot afford to lose and to turn back the tide of isolationism that was rising in our party," he said.

The 60-year-old senator from South Carolina never gained traction, often polling at just one per cent nationally.

In a crowded race that at one point had 17 candidates, Graham was swamped by political outsiders including Trump and retired neurosurgeon Ben Carson, as well as fellow Republican senators Ted Cruz, Marco Rubio and Rand Paul, three higher-profile lawmakers.

"I've hit a wall," he acknowledged in an interview with CNN, and said it was time to consider "getting out and helping somebody else" in the presidential race.

Graham's interventionist foreign policy has been at the fore of his candidacy, and he has helped shape the conversation on the campaign trail, particularly when it came to national security.

In the first in a series of Republican primary debates, he stated that any candidate who did not understand that the United States needed more troops on the ground in Iraq and Syria to defeat ISIS (Islamic State in Iraq and Syria) was not ready to be commander in chief.

"At that time no one stepped forward to join me. Today, most of my fellow candidates have come to recognize this is what's needed to secure our homeland." Graham often touted his experience in the Middle East - he says he has made 36 trips to the region - and said the next US president must have a deft understanding of the conflicts there.

He has expressed particular bitterness towards party frontrunner Trump, describing him as a xenophobic bigot for his comments about Muslims and at one point this month saying American voters should tell the bombastic billionaire to "go to hell".

"Donald Trump has done the one single thing you cannot do: declare war on Islam itself," Graham said during last week's Republican debate.

Graham hails from South Carolina, a conservative southeastern state that holds the third presidential primary of the nominations contest, on February 20 after Iowa and New Hampshire. Should he choose to endorse a candidate before then it could prove critical in his state's primary.

Graham's onetime rivals heaped praise on the senator.

"Nobody is more clear-eyed about ISIS," former Florida governor Jeb Bush, whose father and brother were both presidents, posted on Twitter. "As he leaves the race I hope our party and country listen to his counsel."