Biden was joined by his wife, Jill, and the couple’s granddaughter Natalie. Jill and Natalie held hands for much of the ceremony. Biden was the first to speak and kept his remarks brief. Addressing Gold Star families in attendance, Biden noted that it was the anniversary of Beau’s death. “Four years ago today, we lost Natalie’s dad,” he said. Still speaking to the Gold Star families, Biden said: “We all know the loss of a loved one — somehow the pain fades a little bit, but those moments when we remember are bittersweet because they’re the days when everything comes back. The pride as well as the pain.” A little while later, several members of the Delaware National Guard, in which Beau had served — including about a year in Iraq — stepped forward to place a flower on the wreath that would be laid in front of the monument. As they passed in front of Biden, I thought I saw his jaw muscles tighten.

Ted Kaufman, Biden’s close friend and an informal adviser to the campaign, told me that Biden would have run for president in 2016 had Beau not fallen ill. Donilon had written a 25-page memo outlining Biden’s path to the Democratic nomination and then the White House. But Biden couldn’t make a decision until Beau’s situation was “resolved,” as Kaufman delicately put it, and by the time Beau died, in May 2015, it was too late — there was no way logistically at that point to be competitive with Hillary Clinton. (It has been reported that Obama actively discouraged Biden from seeking the presidency in 2016. In “Promise Me, Dad,” Biden notes that Obama asked him if he intended to run and was “not encouraging.”) But the determining factor was the emotional toll from Beau’s death, a point Jill Biden made when she and I spoke by phone on the morning of the Miami debate. “As Joe has said, when you run for president, your whole heart and your whole soul has to be in it, and we just weren’t there because of Beau’s death,” she told me. Beau was part of the discussion as her husband weighed a decision for 2020. “We talked to one another and thought we were ready,” she said. As she put it: “It’s something you wake up to every morning. It never leaves you.”

When I mentioned Beau in our conversation in June, a stricken expression fell across Biden’s face. He said that contrary to what many people assumed, the promise he made to Beau wasn’t a promise to run for president — rather, when Beau knew that he was dying, he made his father promise that he wouldn’t crumble in grief and withdraw from life. “He knew how much I adored him, and he was worried that I might just drop out,” Biden said. He looked down at the table as he spoke and twisted his phone around in his hands like it was a Rubik’s Cube. “He strongly thought I should run for president,” he said. “That wasn’t the promise. The promise was to stay engaged.”

“What stunned me,” Biden went on, “was he had come to grips with his own death. God, he was an incredible guy.” He said that Beau was now his lodestar. “Almost every morning I get up, I think to myself, I hope he’s proud of me,” Biden said. “I know he’s still here. I know he’s still with me. I really do. I really do.”

Though Biden can be careless with his words, he is usually very disciplined in how he talks about Beau — he invokes his son not to gain sympathy but rather to express solidarity with others who have suffered loss. Still, there is some risk in making Beau’s death part of the narrative of his campaign — it can seem exploitative. Cynics will inevitably see it as a ploy to insulate himself from criticism, or to at least force his opponents to go easy on him. During a recent interview with CNN’s Chris Cuomo, Biden said he was surprised by Kamala Harris’s broadside in Miami because “she knew Beau” — who had been Delaware’s attorney general — a comment that could be construed as an expectation that his son’s death would grant him a degree of immunity.

It certainly won’t against Trump. His campaign has already signaled that it intends to make an issue of Hunter Biden, whose business ventures during the Obama administration have drawn scrutiny, if no evidence of impropriety on his father’s part. Hunter’s complicated personal life, which includes a messy divorce and admitted drug use, will be additional fodder for the Republican Party. And it seems that even Beau will not be out of bounds. At Trump’s re-election kickoff rally in Florida last month, his son Donald Jr. mocked Biden’s initiative to cure cancer. “Wow, why the hell didn’t you do that over the last 50 years, Joe?” the younger Trump sneered, eliciting laughter.

It is an axiom of American politics that campaigns must project a spirit of optimism. But the 2020 race is taking place against a backdrop of deep pessimism and a profound sense of loss. From the gutted middle class to America’s diminished stature, loss is the subtext to this election — and perhaps no political figure in American history has experienced loss as Biden has. During campaign appearances, Biden hits the requisite morning-in-America note, telling audiences that he has never been more optimistic about the country. But he often adds a qualifier — “I know everyone says I’m too optimistic,” he said this month at a rally in Portsmouth, N.H. — that suggests he’s aware the audience probably doesn’t feel the same way.