WASHINGTON — US President Joe Biden's decision to end his re-election campaign on July 21 followed a series of strategic missteps by his campaign team and White House aides that amplified concerns that the 81-year-old president wouldn't be able to win in November's election, or run the country for another four years.

Biden's halting performance at the June 27 presidential debate against Republican candidate Donald Trump, pushed even some of his closest allies to question whether he could endure a full campaign, and added gasoline to a smouldering Democratic party movement questioning the wisdom of his second term.

Within days, Biden went from the party's figurehead to a liability. He became the first sitting president to stand down from a possible re-election since Lyndon B. Johnson in 1968.

These were some of the key moments that led to Mr Biden's downfall.

The debate coup: April to June

Biden himself set the debate in motion, after he told radio show host Howard Stern in April that he'd debate Trump, despite worries from some of his Democratic allies that a debate might do little to improve his chances or even cast an unflattering light on him.

Some Biden aides thought they had scored a coup in agreeing to a June debate. They worked with broadcaster CNN to shape the rules in what they thought was their favour and agreed on May 15 to two debates, including the first on June 27.

An earlier debate would serve Biden well, some aides thought, convincing voters that there was no other anti-Trump option in the race, but also cushioning the blow of any mediocre performance by the president.

The format — no audience to pump up Trump's punchlines, no third-party candidates, moderators they could trust and a mute button on the microphones- would, they thought, favour Biden's style.

Jet lag - June 4 to 27

Biden flew to Europe twice and to the West Coast over a 14-day period before taking just a few days to rest at his vacation home in Rehoboth Beach, Delaware. 

He was tired and dragging by the end of the travel, according to several people who observed him during this period.

He and his aides headed to Camp David for focused prep sessions on June 21, where they peppered Biden with details, then followed up with mock debates for the better part of seven days.

A bad night - June 27

In CNN's Atlanta studios, Mr Biden stumbled over his words and lost his train of thought. His delivery, appearance and voice drew the immediate attention of reporters, who asked his aides for an explanation.

Officials told reporters as the debate was underway some new information: Biden was sick with a cold, they said.

Biden’s hoarseness would improve as the debate went on but his disjointed answers took voters, donors and Democratic officials by surprise. One Democratic strategist called it “a disaster”.

Trump, 78, repeated a series of well-worn, glaring falsehoods during the 90-minute debate, including claims that he actually had won the 2020 election. Biden failed to refute them, leaving sentences and thoughts unfinished.

Trump jabbed Biden for being incoherent: "I really don't know what he said at the end of that sentence. I don't think he knows what he said."

Fallout - June 27 to July 2

"I know I'm not a young man, to state the obvious," Biden said the day after the debate, admitting a poor performance. "Nearly fell asleep on stage," he said on July 2.

Calls for Biden to step down would start in the hours after the debate. Calls from Biden aides to worried Democrats facing reelection campaigns who saw their political futures flashing before their eyes, wouldn't start until days later.

Some lawmakers started to break ranks, starting with US Representative Lloyd Doggett on July 2, and then gathering steam.

Angry donors told Mr Biden's aides they would need to see a turnaround in the candidate's performance. Senior Democrats and Biden allies also began to hint at changes to the ticket.

House of Representatives Democrat Jim Clyburn — a kingmaker of sorts within the Democratic Party who was instrumental in Biden's 2020 win — said on July 2 that he would back Vice-President Kamala Harris if Biden did depart and suggested the idea of a "mini-primary" if Biden stepped aside.

Former House Speaker Nancy Pelosi, a longtime Biden ally, said it was legitimate to ask whether Biden's performance in Atlanta was an "episode" or a condition. She also hinted that Biden would need to examine his choice to say in the race.

The dam breaks - July 5 to 8

Mr Biden would fail to quiet the chorus of dissent.

In his first major interview after the debate, on July 5, Biden told ABC News anchor George Stephanopoulos that only the "Lord Almighty" could cast him from the race.

More worrying to some Democrats, Mr Biden said he could accept remaining in the race and losing to Trump “as long as I gave it my all and I did as good a job as I know I can do”.

Thirty-six Democratic US lawmakers and one independent who caucuses with the Democrats would ultimately call for Biden to step off the ticket over the 24 days that followed the debate.

Nato, polls - July 9 to 12

Biden tried to move on. He gave interviews, held a press conference and gave stern speeches on the campaign trail as well as at the Nato summit of US allies.

But the events sometimes raised more concerns than they quieted.

At the Nato summit during the second week of July, Biden mixed up the names of his vice-president Harris and his Republican rival Trump, and those of Ukrainian President Volodymyr Zelensky and Russian President Vladimir Putin, whose countries are at war with each other.

Polls showed Biden trailing other Democrats in many of the key states and districts Democrats need to win in November, though polls nationwide continued to point to a close race.

Biden was unmoved, still believing he was the best candidate to face Trump, that he could rebound from this setback as he has so many times before, a view reinforced by a close circle of aides.

Trump rally shooting - July 13

Trump was shot while giving a speech in Pennsylvania. The bullet grazed his ear, bloodied his face, and photos of the former president holding up his fist in defiance were shown widely.

Republicans spoke of divine providence, unifying around their candidate. Democrats worried if their own chances were cursed, seeing only a narrower path to any victory in November.

Days later, Biden got Covid-19 while campaigning in Nevada. Recuperating at his Rehoboth Beach, Delaware, home, he would be left with time, alone and in isolation, to decide whether and how to end his campaign.

Source: Reuters