Republican presidential candidate Jeb Bush on Friday won the endorsement of former rival Lindsey Graham, then turned his fire on the competitor who once had common ground with them on immigration: Marco Rubio.

“Marco cut and run,” Bush said at a news conference in Graham's South Carolina. “What kind of leader will we have if the first impulse is to cut and run?”

Graham and Rubio helped put forward a 2013 Senate bill that would have created a pathway to citizenship for some people living in the U.S. illegally. The measure stalled in the Republican-led House and Rubio backed away from it in the face of conservative ire. 

Immigration has become one of the top issues in the presidential race, as hard-liner Donald Trump, who promises to build a wall along the U.S.-Mexico boarder, dominates early-state and national polls. Bush, who has called for more compassion into the immigration debate, flounders in those surveys.

Graham said his Senate primary victory in 2014 shows Republicans can support immigration reform and still win in South Carolina.

“The thing I like about Jeb is that he hasn't run away,” Graham told reporters.

Bush said Rubio, a fellow Floridian, sought his support on the immigration bill, and Bush gave it “even though my ideas were clearly different, particularly on the path to legalization.”

“He asked for my support on a bill, and he cut and run,” Bush said about Rubio. “And he cut and run on his colleagues as well.”

Rubio says immigration has become a “dramatically different issue” due to the rise of the Islamic State—“a group of radical crazies,” he said Thursday during the Republican presidential debate at the North Charleston Performing Art Center, just a short walk from where Graham and Bush spoke on Friday. IS has “a sophisticated understanding of our legal immigration system and we now have an obligation to ensure that they are not able to use that system against us.”

Rubio, who has been endorsed by U.S. Representative Trey Gowdy of South Carolina, released a TV ad earlier on Friday in which he says, “Jeb Bush is desperate, and spending millions on false attacks.

“We'll add 20,000 border agents, finish all 700 miles of border wall, and if we aren't 100 percent sure who you are and why you're coming to America, you're not getting in,” Rubio says in the spot.

“When Marco is president, there will be no amnesty and we will secure the border,” Rubio spokesman Joe Pounder added in a statement after the endorsement, accusing Bush of holding “ever-changing immigration positions.”

Bush is seeking a foothold in South Carolina, which is set to hold the south's first primary election on Feb. 20, and the hawkish Graham's national security credentials could help him there. The winner of the South Carolina Republican primary has gone on to win the party's nomination in every election but one since 1980—2012, when former House Speaker Newt Gingrich won the state before former Massachusetts Governor Mitt Romney clinched the standard-bearer title. 

Graham, who left the race in December and is the first former 2016 candidate to endorse, said South Carolina would give Bush the momentum to win the nomination. 

“Jeb Bush is ready on day one to be a commander-in-chief,” Graham said at the news conference, adding he couldn't “think of a worse idea” for how to win the fight against the Islamic State than front-runner Donald Trump's proposed ban on Muslims entering the U.S.

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