Timeline: The end of Biden's 2024 presidential campaign

Watch the ABC News special, "Biden Stands Down: The Race for the White House," tonight from 7-9 p.m. ET on ABC and ABC News Live.

President Joe Biden has dropped out of the 2024 presidential race after his poor performance in a June debate against Donald Trump led to questions from the Democratic party and others about his age, cognitive health, his ability to beat Trump, and whether he could serve a second term.

Here's a look at how the Biden campaign reached this historic decision to end Biden's candidacy for a second term.

The first debate

Biden, who at 81 is the oldest sitting president in history, had already been facing concerns about his age before the CNN debate against Trump on June 27. However, Biden's debate performance increased questions not only about his ability to win against Trump, but his ability to execute his duties as president for another term.

The president's performance -- marred by a hoarse voice, often meandering answers, and a lack of strong attacks against Trump on key issues -- left some Democrats calling for him to step down.

Polls prompt further questions

The debate quickly hurt Biden's campaign. The two candidates were just about tied on the day of the debate, according to 538's national polling average. After the debate, Trump led Biden by two percentage points.

Amid the scrutiny, Vice President Kamala Harris, whom Biden endorsed as his party's nominee in Sunday's statement announcing the end of his candidacy, had already emerged as a likely top contender to replace Biden.

In an Ipsos poll about a potential matchup between Harris and former President Donald Trump, released on July 2, Harris and Trump were nearly tied, with 42% supporting her and 43% supporting the former president

Another poll released by CNN in the first week of July found that a matchup between the vice president and Trump resulted in a 45-47% split between Harris and Trump, compared to the 43%-49% split between Biden and Trump.

Yet in a poll released July 11, which asked voter preferences if Vice President Kamala Harris were to replace Biden as the nominee, voter choices were 49-46%, Harris-Trump, among all adults and 49-47% among registered voters. Harris’ 49% was slightly better than Biden’s 46%, although it wasn't a statistically significant lead over Trump.

Biden defends campaign, explains performance

President Joe Biden, in his first television interview following the debate against Trump, brushed off his poor performance.

"It was a bad episode," Biden told ABC News' George Stephanopoulos on July 5. "No indication of any serious condition. I was exhausted. I didn't listen to my instincts in terms of preparing and -- and a bad night."

Biden also said he was under the weather and "feeling terrible."

"I asked if they did a COVID test because they're trying to figure out what was wrong. They did a test to see whether or not I had some infection, you know, a virus. I didn't. I just had a really bad cold," the president said.

When Stephanopoulos asked: "If you can be convinced that you cannot defeat Donald Trump, will you stand down?"

Biden responded: "It depends on if the Lord Almighty comes down and tells me that, I might do that."

In a nearly hourlong solo press conference on July 11, Biden insisted that he's best situated to defeat former President Donald Trump this November, and said he was staying in the race despite his shaky debate performance.

When asked if he would consider dropping out if his team showed data suggesting he could lose, he said: "No, unless they came back and said, 'There’s no way you can win' …. No one is saying that. No poll says that."

Trump's assassination attempt and the RNC

Former President Donald Trump was shot in the ear in an assassination attempt at an election rally in Butler, Pennsylvania, on July 13.

Following the shooting, as well as the Republican National Convention the following week, former President Donald Trump's favorability rating among Americans rose, while a majority of Americans wanted President Joe Biden to drop out of the race, according to an ABC News/Ipsos poll.

Trump's favorability rating increased to 40% following last week's events, marking his highest favorability rating in four years of ABC News/Ipsos polling. For most of the last four years, it has hovered in the low- to mid-30% range.

Its high over the last nine years was 42% in August of 2020 according to an ABC News/Washington Post poll conducted then. At the same time, about half of Americans, 51%, currently view Trump unfavorably, according to this most recent poll.

Biden campaign scrambles amid questions

ABC News reported that Biden's support from party leaders began to crumble as reports that former House Speaker Nancy Pelosi, Senate Majority Leader Chuck Schumer, and other top politicians spoke with him and his campaign about the worsening reelection outlook.

On July 11, Biden's campaign began testing head-to-head matchups of Vice President Kamala Harris against former President Donald Trump, a source familiar with the strategy told ABC News.

Pelosi spoke with President Joe Biden around the same time, telling the president that she and other members of the Democratic Party were concerned about him staying in the race, a source confirmed to ABC News.

Schumer also had a meeting with Biden earlier this month. A knowledgeable source close to both men told ABC News that Schumer made the case that it would be best if Biden dropped out of the 2024 presidential race.

However, on July 19, Biden said he would be returning to the campaign trail to continue to take on Trump following his COVID-19 diagnosis.

"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 said in a statement.

According to a new ABC News/Ipsos poll released on July 21, 60% of Democrats believe President Biden should drop out of the race for reelection.

Biden drops out in historic move

Biden said in a statement posted Sunday that he will "stand down."

"It has been the greatest honor of my life to serve as your President," he wrote, in part, in a letter posted on X. "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."

In another post on social media, Biden gave his "full support and endorsement" for Vice President Kamala Harris to be the Democratic Party's nominee.

ABC News' Tal Axelrod, Gary Langer, Ivan Pereira contributed to this report.