Senator Lindsey Graham ends US presidential bid

WASHINGTON: Lindsey Graham, the hawkish Republican senator who has called for 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 centerpiece 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.

Republican candidates talk tough on 'radical Islam' after California attack

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

In a race crowded at one point with 17 candidates, Graham was swamped by political outsiders including Trump, retired neurosurgeon Ben Carson and to a degree his fellow Senator Ted Cruz.

But 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 the Islamic State group 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 recognise 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 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."