Lindsey Graham was just 22 when his parents died 15 months apart, leaving him to raise his sister Darline, then 13, on his own.

He put his own life on hold to bring her up, later adopting her so that the teenager could draw on his military benefits.

Darline tearfully recalled the day they were left orphaned in a recent ad for Graham's re-election campaign. 

'Lindsey came running in and just grabbed me up and hugged me. He was like, "I am so sorry, but it's going to be OK. I’m going to take care of you." And he did. He’s always been there for me.'

Darline declined an interview Monday, telling the Daily Mail it was 'too difficult' to speak about her brother so soon after his death.

Graham, 71, was a lifelong bachelor, and his sister and her two daughters were his only close family. Preliminary findings from the DC medical examiner's office found he died of an aortic dissection caused by cardiovascular disease after paramedics were called to his Capitol Hill home Saturday night for chest pains.

He told the Daily Mail in 2015 during his presidential run that Darline could serve as first lady if he ever made it to the White House.

'I've got a sister, she could play that role if necessary,' he said, adding with a chuckle, 'I've got a lot of friends. We'll have a rotating first lady.'

Now Darline, 63, has entered politics in the most tragic circumstances, named South Carolina's interim senator with Donald Trump's blessing.

A Republican primary in August will decide who takes the seat when Graham's term expires in January.

'This would be a fabulous tribute to Lindsey, who loved her dearly,' Trump posted on Truth Social.

South Carolina Governor Henry McMaster appointed Darline at a press conference later on Monday.

Darline told reporters it was 'a privilege' to help finish Graham's 'important work.'

'My brother was the most amazing person, outstanding leader, and just a genuinely good man,' she said.

'I think this is what Lindsey would have wanted, and I plan to honor him in this way.'

The siblings grew up sharing a single room behind the Sanitary Cafe, the bar, pool hall and liquor store their parents ran on Main Street in the small town of Central, South Carolina.

The family later moved into a trailer and then a house next door to the bar.

Darline leaned on her big brother long before they were orphaned, running to him whenever she was sick or stuck on her homework.

He taught her to ride a bike, sprinting alongside and shouting encouragement, and picked her up when she fell.

'He's kind of like a brother, a father and a mother rolled into one,' she told the New York Times in 2015.

Their mother died of Hodgkin's lymphoma in 1976, and a heart attack claimed their father a little more than a year later.

Darline moved in with an aunt and uncle while Graham finished his studies at the University of South Carolina.

He gave up his weekends to travel home and care for her while his classmates stayed on campus to enjoy themselves.

He then enlisted in the Air Force, where his guardianship of Darline allowed her to qualify for his benefits, and she has credited him with getting her through high school and on to college.

She went on to become a mother of two and built a career helping people with disabilities find work, a success her brother wore as a badge of honor.

'Of all the things that have happened in my life, her turning out so well is the highlight of it by far,' Graham told C-SPAN in 2015.

Darline introduced him on stage when he launched his White House bid in their hometown in 2015.

'Lindsey was always my parent,' she said at the time. 'There was no doubt in my mind or anyone else's mind that Lindsey was my guardian.'

She has said his devotion to raising her was the reason he never married.

'He was just dedicating all of that time to raising me and going to school and trying to get an education. There's just only so much time in a day,' she told the Times.

Fifty years after he promised to take care of her, Darline has been appointed his successor, the sister he raised now serving as caretaker to his constituents of more than two decades.

'I promise to work hard over the next several months to support the President and carry forward the efforts of my brother on behalf of the citizens of South Carolina and the United States,' Darline said.