The seeds of President Biden’s reaction to the Democratic Party’s crisis of confidence in him were planted years ago.

Facing an extraordinary push by his friends and allies to end his political career, Mr. Biden has responded with defiance. For three weeks he has dug in, denied polling evidence and vowed to stay in the race, claiming that he is the only one who can defeat former President Donald J. Trump. Only in the last few days have people close to him said they believe he is more receptive to stepping down.

To many of his allies, he is in a surprising crouch for a politician who has built his identity on service to country over self.

But those who have been close to him for years say that Mr. Biden’s response is a culmination of the regret, pride, anger and fear that has been building in him for at least the last decade. All of that is painfully evident, they added, from his own words.

It was in October 2015 when Mr. Biden, then vice president and grieving over the loss of his son Beau five months earlier, announced in the White House Rose Garden that he would not run against Hillary Clinton and Senator Bernie Sanders for the Democratic presidential nomination. A few months later he looked back on the decision and said, “I regret it every day.”