What pushed President Biden to withdraw from the reelection race?

Just two days after announcing he was prepared to return to the campaign trail, President Joe Biden announced Sunday that he is dropping out of the race, taking members of his staff by surprise.

"It has been the greatest honor of my life to serve as your President," Biden said in part in his statement.

TIMELINE: The end of Biden's 2024 presidential campaign

Biden's decision to withdraw from the reelection campaign came in the wake of repeated White House statements, and declarations from Biden himself, that he would remain in the race and was confident that he could defeat former President Donald Trump's reelection bid. Yet those assurances did little to decrease the public pressure from an increasing number of his fellow Democrats for Biden to move aside in favor of a stronger candidate.

Some 48 hours after Biden, who is recovering from COVID-19, said he would return to the stump this week, the pressure from those in his own party for him to withdraw from the race continued to mount.

Not only did the list of Democratic congressional members echoing the call for Biden to withdraw grow to at least 40, staunch Biden supporters like Sen. Joe Manchin, I-W.Va., upped their public calls for the 81-year-old president to end his presidential campaign.

Just before Biden announced that he was stepping aside and endorsing his vice president, Kamala Harris, to lead his party's ticket, Manchin went on ABC's "This Week" and encouraged Biden "to pass the torch to a new generation."

"He will go down with a legacy unlike many people, as one of the finest and truly a [patriotic] American," Manchin said. "So, with that, I come with a heavy heart to think the time has come for him to pass the torch to a new generation."

Biden's decision came just eight days after Senate Democratic Leader Chuck Schumer visited Biden in his Rehoboth, Delaware, beach home and had a heart-to-heart talk with Biden about leaving the race, sources told ABC News Chief Washington correspondent Jon Karl.

Sources further told ABC News that Biden's own staff didn't know Biden was stepping down until just one minute before he publicly announced his decision on Sunday, leaving some members of the president's staff blindsided by the decision.

RELATED: Joe Biden just dropped out. What happens next?

Vice President Harris did not say in a statement she released Sunday when she learned Biden was stepping aside, focusing instead on praising Biden's leadership.

"On behalf of the American people, I thank Joe Biden for his extraordinary leadership as president of the United States and for his decades of service to our country," Harris said. "His remarkable legacy of accomplishment is unmatched in modern American history, surpassing the legacy of many presidents who have served two terms in office."

In a letter to the nation, Biden didn't elaborate on what thinking may have prompted his decision, saying only "And while it has been my intention to see 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."

Biden's statement also said he would speak later this week to the nation about his decision, which came came 24 days after his disastrous June debate with former President Donald Trump.

Just two days prior to Sunday's announcement, Biden sounded as if he had no plans to withdraw from the reelection race. In a statement Friday, the president said, "I look forward to getting back on the campaign trail next week to continue exposing the threat of Donald Trump's Project 2025 agenda while making the case for my own record and the vision that I have for America: one where we save our democracy, protect our rights and freedoms, and create opportunity for everyone."

Biden's youngest brother, Frank Biden, told ABC News that his brother's subsequent decision to withdraw was a "close call" reached only after several Biden family members held several discussions over the past week, led by first lady Jill Biden, and that the family has always been supportive of the president doing whatever he thought best for the country.

"It really boils down to this: it has to do with his overall health and vitality. Not anything cognitive. But he's a proud guy, it pisses him off that he shuffles when he walks," Frank Biden told ABC News.

Frank Biden also said it has "been really horrible [for the family] to see the dogs let loose on him," referring to longtime allies who in recent weeks joined the calls for Biden to drop out of the race.

"Joe has one thing uppermost in his mind. It's not him, not his ego, not his ambition. He has really reached a place where the only thing that matters to him is the health of this nation," Frank Biden said.