NORTH CHARLESTON, S.C. — Sen. Lindsey O. Graham (R-S.C.) on Friday endorsed former Florida governor Jeb Bush for the GOP presidential nomination, saying, "Jeb Bush is ready on day one to be commander in chief."

Graham, who recently ended his own campaign for the nomination, made the endorsement just seconds away from the site of Thursday night's presidential primary debate, flanked by veterans who had already endorsed Bush.

"I have concluded without any hesitation, without any doubt, that Jeb Bush is ready on day one to be commander in chief," said Graham. "He understands that America can't go it alone, but of all the others, he understands how to bring the world on board."

Graham, who had attended the debate, praised Bush for grappling with Donald Trump on the front-runner's proposal to temporarily halt all Muslim immigration to the United States.

"Last night I heard from Jeb Bush the right answer," said Graham. "We cannot and should not declare war on a religion."

Promising that South Carolina's primary would "reset this race" -- a veiled reference to Bush's swoon in the polls in Iowa and New Hampshire -- Graham mentioned no other candidates. But he did subtly criticize the Republicans who had embraced immigration reform after the 2012 election, only to reject it after conservative backlash.

"The thing I admire most about Jeb is that he stayed true to who he is, that he hasn't tried to get ahead in a contested primary by throwing dangerous rhetoric around," said Graham.

In a short news conference after the endorsement, however, Bush responded to new super PAC ads supporting Florida Sen. Marco Rubio by saying his one-time protege had "cut and run" when the immigration fight got too nettlesome.

"Marco cut and run, plain and simple, for whatever reason," Bush said. "There may be legitimate reasons, but he cut and run. He asked for my support on a bill and he cut and run. He cut and run on his colleagues as well. That bill passed the Senate. Who knows what would have happened in the House. There probably would have been a view that was more closely mirroring mine, and then you would have gone to conference, or who knows."

Graham declined to criticize Rubio, one of his Gang of Eight partners on the 2013 immigration bill, on how he handled its aftermath.

"I think Marco Rubio will be president of the United States one day," said Graham. "I think he's one of the most gifted people I've ever met. I like him. But I wasn't ready to be president at 44."

After Bush left the room, Graham stuck around to tell reporters that they did not agree on every aspect of immigration reform, but that the Floridian was the sort of leader who could listen to what worked.

"I feel confident he's got the right attitude about immigration," he said. "I think he's open-minded to a pathway to citizenship if that's the will of the Congress."

Bush, who had remained in at least weekly contact with Graham when the two men were competing in the primary, praised the senator for focusing his race on foreign policy and the threat of the Islamic State.

"I am honored to be your student," said Bush. "I will continue to seek out your advice."

Whether the Graham endorsement can help deliver new momentum and better support in South Carolina and elsewhere remains unclear. But it will allow Bush to continue making the case that he's an experienced, steady alternative to Trump and Sen. Ted Cruz (R-Tex.).

Sen. Lindsey Graham (R-S.C.) announced he is suspending his presidential campaign in this video released on Dec. 21. (Youtube/Senator Lindsey Graham)

Graham launched his long-shot presidential bid last year in hopes of making foreign policy and national security a central theme of the campaign. But he never climbed out of the single digits — even in his home state — and left the race last month to return full time to the U.S. Senate.

Once Graham dropped out, dozens of his South Carolina supporters and top donors quickly fell in line with Bush, leaving many to believe that the senator would eventually back the former governor. And Bush, like many other GOP presidential candidates, reached out to Graham the day he dropped out of the race in December and has been working to secure his support ever since, aides said.

David Wilkins, a former U.S. ambassador to Canada and South Carolina GOP lawmaker who is backing Bush, said ahead of the announcement on Thursday that Graham "has got a sound organization ."Many of his supporters, not all of them, but many have gone to the Bush campaign and are supporting him now. But he’s got a good organization and his endorsement would be, could be a big difference."

One Bush adviser in South Carolina, who asked for anonymity to speak frankly about campaign strategy, said that the influx of Graham support in recent weeks has been a "huge help" across the Palmetto State.

"It expands our statewide network of grass roots and grass tops across the state in terms of organizing and fundraising," the aide said.

O'Keefe reported from Washington.