The nomination of Kamala Harris in Chicago on Thursday to be the Democratic Party's standard-bearer in the November presidential election will cap one of the wildest months in US political history.

On the afternoon of July 21, President Joe Biden upended the White House race with a post on X saying he would not seek reelection.

The 81-year-old Biden bowed to growing pressure from party leaders to step aside following his disastrous debate performance against Republican candidate Donald Trump in late June.

Biden's announcement, while not completely unforeseen amid mounting concerns about his age and mental fitness, sent shockwaves through a country still reeling from the attempted assassination of Trump just one week earlier.

- Harris endorsement -

Twenty-seven minutes after his first message, Biden threw his support behind Harris, endorsing his 59-year-old vice president to be the party's presidential nominee in the November 5 contest against Trump.

"My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President," Biden said on X. "And it's been the best decision I've made.

"Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year."

- 'Honored' -

Harris -- now vying to be the first woman president of the United States -- thanked Biden for his "extraordinary leadership" and said she was "honored" to have his backing.

She pledged to do "everything in my power to unite the Democratic Party -- and unite our nation -- to defeat Donald Trump."

Governors, senators, former rivals, progressives and moderates lined up behind her candidacy within hours in a remarkable display of party unity.

Harris's husband, Doug Emhoff, was not among the first to learn that his wife had been catapulted to the forefront of the White House race.

Emhoff was in a SoulCycle exercise class in Los Angeles when the news broke of Biden's withdrawal and his endorsement of Harris.

Upon recovering his mobile phone, Emhoff found he had missed a deluge of calls, including several from his wife.

"Never leaving my phone in the car again," he joked.

- 'When we fight, we win' -

With the election just three months away, Harris threw herself headlong into the campaign, and has taken up the slogan "When we fight, we win."

"We are going to take our case to the American people, and we are going to win," Harris told campaign staff in Delaware, acknowledging the recent "roller coaster" of "mixed emotions."

Harris kept on Biden's campaign staff and organizational structure, working out of the same offices with virtually the same logo.

But the message has completely changed.

Harris has sought to contrast her experience as a former prosecutor and attorney general of California with Trump's status as the first former US president ever convicted of a crime.

Harris has also noted their age difference -- Trump is nearly 20 years older than her -- taking away one of the Republican candidate's main attack lines against Biden: that the president was allegedly declining physically and mentally and too old for the job.

Harris's entry into the race led to an influx of donations and she raised $100 million in the 48 hours after she was moved to the top of the Democratic ticket.

The enthusiasm generated by her candidacy has been evident not only in opinion polls but also in the huge crowds she has attracted to her campaign rallies.

- Vice president pick -

In early August, Harris effectively secured the Democratic Party's presidential nomination after a five-day electronic vote of nearly 4,000 party convention delegates.

With the nomination in hand, Harris began a search for a vice president and made a decision within days, condensing a painstaking process that normally takes months.

Her choice, unveiled on August 6, was Tim Walz, the 60-year-old governor of Minnesota.

Walz, a former high school geography teacher and football coach, and Harris immediately embarked on a weeklong tour of five battleground states likely to be decisive in the November election.

After Harris formally accepts the Democratic nomination on Thursday at the culmination of the party's convention in Chicago, she will embark on what promises to be a grueling campaign run to election day.