Watch CNN’s full exclusive interview with Sen. Lindsey Graham at 11 a.m. ET on “At This Hour”

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Sen. Lindsey Graham told CNN's Kate Bolduan in an exclusive interview that he is ending his presidential campaign

Graham is an experienced foreign policy voice, but his campaign failed to take off despite strong showings at the GOP undercard debates

CNN  — 

Senator Lindsey Graham is ending his presidential campaign, he told CNN during an exclusive interview airing Monday.

“I’m going to suspend my campaign. I’m not going to suspend my desire to help the country,” the South Carolina senator said in a wide-ranging and candid discussion in which he acknowledged: “I’ve hit a wall here.”

He made the official announcement in an email to supporters and Youtube video posted Monday morning.

Graham is known for his quick wit and famous for his one-liners (just ask Princess Buttercup about his retort from the last debate), but he was sober, serious and emotional as he described his decision to leave the race just weeks before the voting begins.

One thing is clear: Graham still wants his voice heard on the direction his party is headed, especially with regard to the Middle East.

“Here’s what I predict. I think the nominee of our party is going to adopt my plan when it comes time to articulate how to destroy ISIL,” he said. “We’ve fallen short here, but the fight continues. To those who are doing the fighting, I want to be your voice. To those in the Republican Party who want to win, check my plan out. Hillary, if you get to be President, I’ll help you where I can. I hope you’re not. But if you are, I’ll be there to help you win a war we can’t afford to lose.”

Graham’s decision – which leaves a field of 12 main GOP candidates (CNN had 13 at its last Republican debate, including Graham) – comes just days after the CNN Republican Presidential Debate in Las Vegas, where he was widely viewed as dominating the undercard debate.

However, Graham was never able to break onto the main stage after announcing his candidacy in early June — a reality of the Republican Party’s new debate structure that he blames at least in part for his struggles. Graham limped along at less than one percent in national polls, and, in the most recent CNN/ORC national poll, he barely registered. He failed to even qualify for the undercard round in the Fox Business debate in November.

“Four months ago at the very first debate, I said that any candidate who did not understand that we need more troops on the ground in Iraq and Syria to defeat ISIL was not ready to be Commander in Chief,” Graham wrote in the email. “At the 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.”

He added, “While we have run a campaign that has made a real difference, I have concluded this is not my time.”

Related: Lindsey Graham’s best lines from the CNN debate

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Graham: 'If you want to win this war, follow me'
01:35 - Source: CNN

“My biggest problem is a lot of people like what I say, but not a lot of people hear it,” he told CNN. “I don’t want to be the undercard voice. I cannot tell you how frustrating it has been to have spent all this time and effort preparing myself to be Commander-in-Chief and to be put at the ‘kiddie table’.” His advice to his party as it relates to the debate format: “Never do this again.”

Graham’s campaign strategy had been laser-focused on New Hampshire: Place high in that first primary state on February 9 and use the momentum to propel him forward. So why not hold out until then?

“I’m not trying to hold out. I’m trying to make a difference. I think the best way for me to make a difference is to think about helping somebody else,” he said.

In another key primary state, South Carolina, he faces a Monday December 21 deadline to keep his name off the ballot, potentially helping Graham avoid an embarrassing showing in his home state. Asked whether that fueled his decision to drop out now, Graham said no and added that he’d risk it if he saw a way to turn things around.

“At the end of the day, I’m not going to be competitive in my state,” he said. “I’m not going to be competitive outside my state.”

Asked who he would endorse, Graham demurred. “I’m going to take some time with my family. Going to think about what I should do. I have no intention of endorsing anyone right now.” Later, he said that if he were to endorse, he’s looking for the candidate best fit to be Commander-in-Chief and also who can win.

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Graham: Trump is a 'race-baiting, xenophobic' bigot (2015)
03:34 - Source: CNN

As for the candidate Graham has traded barbs with the most since he entered the race in early June, Graham struck a markedly softer tone toward Donald Trump.”You’re doing really well. I’m impressed with your campaign,” he said.

Graham – who recently urged Republicans to tell Trump to “go to hell” – also did not rule out accepting a Cabinet position in a Trump administration in the future.

But he did offer these parting words for Trump: “We’re at war. A lot of men and women are at risk overseas. Watch what you say over here.”

“You may wind up being the nominee of the Republican Party,” he added. “The Republican Party’s future may be in your hands. The future of the country will be in your hands if you’re President of the United States. This is not a game show. This is not a reality show.”

Candidly, Graham said he has a lot of regrets looking back at his campaign. “I regret that I haven’t been a better candidate. I regret that I never got on the big stage. I regret that I didn’t make it to the final group. But that’s just about me. I have no regrets about running for President,” he said. “It has been the joy of my life to run for President of the United States.”