By now, many are familiar with the tragedy that profoundly impacted Joe Biden's early adult life.

On December 18, 1972, the 30-year-old senator-elect was preparing his transition into office when he learned his family had been in a horrific car accident back home in Delaware. His wife, Neilia, and baby daughter, Amy, were killed, while 3-year-old Beau and 2-year-old Hunter were left with leg and head injuries.

It was an event that nearly derailed a political career as it was getting off the ground, but one that also shaped the distinct relationship between Joe and his surviving sons, particularly the one who inherited the ambition for government service and a gift for connecting with people.

READ MORE: The Heartbreaking Car Accident that Killed Joe Biden's Wife and Daughter

Beau followed his dad's footsteps into public office

As described in his 2008 memoir, Promises to Keep, Joe decided to commute to D.C. and back to Delaware every day, making sure to be there when his sons woke up and went to bed. Many times, one or both boys could be found scampering on the train seats beside "Amtrak Joe" as he made his regular trips to the nation's capital.

The devotion to family life helped all three recover from the life-altering loss and forge an unusually tight bond. A few years later, after teacher Jill Jacobs had become a beloved addition to the group, Beau and Hunter helpfully informed their father that "we think we should marry Jill."

By the time the boys were teenagers at Archmere Academy, Beau was showing signs of following in the elder Biden's footsteps. A natural leader, he was known as "The Sheriff" among friends and was elected student-body president.

After attending Syracuse Law School, his father's alma mater, Beau began carving out his own path as a public servant. He spent five years as a federal prosecutor in Philadelphia, before winning election to the Delaware Attorney General's office in 2006. He also joined the Delaware National Guard in 2003, eventually rising to the rank of major.

Joe Biden with his son, U.S. Army Capt. Beau Biden, at Camp Victory on the outskirts of Baghdad on July 4, 2009

Joe Biden with his son, U.S. Army Capt. Beau Biden, at Camp Victory on the outskirts of Baghdad on July 4, 2009

Beau became a vital campaign surrogate for Joe

Beau also developed into an invaluable campaign presence for the elder Biden. Shortly before being deployed to Iraq with his unit in 2008, Beau made a splash at the Democratic National Convention with a speech that shined a light on his father's impressive efforts as a single parent.

Four years later, Beau delivered another rousing introduction at the Democratic Convention that concluded with him citing his dad as "my hero" and dramatically calling for a vote by acclamation to re-nominate him for VP.

"Beau had a way of instilling courage and calming me," Joe wrote in his 2017 memoir, Promise Me, Dad, noting how his elder son was usually the last person in the room with him before important campaign events.

"Beau would always grab my arm just before I walked onstage and pull me back toward him until I was looking into his eyes. 'Dad. Look at me. Look at me, Dad. Remember, Dad. Home base, Dad. Home base.'"

Beau was diagnosed with aggressive brain cancer in 2013

By 2010, however, the Biden family was already headed for another trial. Beau suffered a stroke that year, and while he was given a clean bill of health in the aftermath, he began suffering from aphasia, a condition in which he had trouble remembering proper nouns.

In 2013, the situation became more dire when Beau was diagnosed with glioblastoma, a particularly aggressive brain cancer. He opted for an experimental treatment and attempted to carry on business as usual, even announcing in spring 2014 that he planned to run for Delaware governor in 2016.

Behind the scenes, the rest of the family watched with concern as Beau grew weaker. In fall 2014, Joe wrote in his later memoir, he was jolted when his son said he was "going to be OK no matter what happens," seemingly acknowledging that he was fighting a losing battle, before adding, "You've got to promise me, Dad, that no matter what happens, you're going to be all right. Give me your word, Dad, that you're going to be all right. Promise me, Dad."

Beau Biden welcomes his dad, Joe Biden, at the Pepsi Center during the third day of the Democratic National Convention on August 27, 2008, in Denver, Colorado

Beau Biden welcomes his dad, Joe Biden, at the Pepsi Center during the third day of the Democratic National Convention on August 27, 2008, in Denver, Colorado

Joe was torn by the decision to run for president following Beau's death

By early 2015, the pressure was on for Joe to announce whether he intended to run for president in 2016. While noncommittal in public, the VP was already leaning toward the affirmative, his family viewing the endeavor as something to give them a common purpose and, hopefully, carry them through to happier days ahead.

But that dream began falling apart as Beau's condition took a turn for the worse in April. The brief recoveries turned out to be illusions, and 46-year-old Beau took his final breath at Maryland's Walter Reed Medical Center on May 30, 2015.

"Beau measured himself as a husband, father, son and brother. His absolute honor made him a role model for our family," the heartbroken father said in a statement shortly afterward. "In the words of the Biden family: Beau Biden was, quite simply, the finest man any of us have ever known."

The second major tragedy of his life presented something of a Catch 22 to Joe, who was too distraught to pursue the Oval Office but also worried about betraying his son's wishes by throwing in the towel. It wasn't until late October that he formally acknowledged the futility of undertaking a presidential campaign while still grieving.

Joe continues to draw inspiration from his eldest son

In the epilogue to Promise Me, Dad, Biden wrote that he came to understand that, more than anything else, Beau wanted him to remain engaged in public service.

Of course, that engagement led right back to the campaign trail with Joe's announcement that he was running for president in 2020 and his rise to the top of the Democratic field.

And while his matchup against Republican incumbent Donald Trump occupied the minds of most voters, Joe relayed how much he was still thinking about his eldest son to the media, including an emotional appearance on Morning Joe in January 2020.

"Beau should be the one running for president, not me," he told host Joe Scarborough. "Every morning I get up Joe, not a joke, and I think to myself, 'Is he proud of me?'"