President Biden has decided to drop out of the presidential race, he announced Sunday, upending the campaign and hurtling Democrats toward a complicated and potentially divisive process to replace him.

The move comes after Mr. Biden’s disastrous performance in the first presidential debate at the end of June, which left many members of his own party questioning whether he was capable of defeating former President Donald Trump in November and serving a second four-year term. Pressure soon grew for him to step aside, with an increasing number of party leaders and Democratic lawmakers openly musing about replacing him on the ticket.

“It has been the greatest honor of my life to serve as your president,” Mr. Biden wrote in a letter to the American people posted to social media. “And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as president for the remainder of my term.”

The decision to step aside has few parallels in American history. The most immediate precedent can be found in 1968, when President Lyndon Johnson declined to run for the Democratic nomination, kicking off a tumultuous process to find a replacement that ended in Vice President Hubert Humphrey becoming the nominee at the party’s convention in Chicago.

This August, Democrats will once again meet in Chicago to decide the path forward.

Follow live updates on the fallout from Mr. Biden’s decision to step aside, and what happens next: