Underneath a picture of herself, the California senator wrote: “I know Joe’s heart, and I’ve seen first-hand his compassion and dedication to public service. He’ll be a president for all of us, and I’m giving everything I’ve got to help him succeed this year.”
It was a further signal, if one were needed, that Ms Harris, 55, is now a clear favourite to become Mr Biden’s running mate. He has said he will make a decision next week. But the message was also clearly intended to defuse a vicious behind-the-scenes battle that has reached fever pitch in recent days.
With so much on the line, the knives have been out for Ms Harris within the Democratic party. Anonymous briefings, some emanating from people in 77-year-old Mr Biden’s own camp, have attempted to portray her as disloyal, ruthlessly ambitious, opportunistic, untrustworthy and intent on becoming president herself.
As one Democrat donor put it: “She would be running for president the day of the [Biden] inauguration. For me, loyalty and friendship should mean something.”
Simultaneously, some Democrats have been furiously talking up the chances of other candidates including Susan Rice, Barack Obama’s former national security adviser, congresswoman Karen Bass, chair of the Congressional Black Caucus, and senator Tammy Duckworth, a military veteran. Much of the internal doubt over Ms Harris goes back to the verbal attacks on Mr Biden she gave when campaigning for the presidential nomination.
During a televised Democrat debate a year ago, and despite having been close to his late son Beau, Ms Harris unleashed on Mr Biden.
She accused him of having opposed “busing”, a policy from the Seventies that was used to desegregate education by transporting black children to mostly white schools.
Ms Harris said she, as a young black girl, had benefited from the policy, but Mr Biden had joined with segregationist senators in an attempt to stop it.
“I do not believe you are a racist,” Ms Harris told Mr Biden, looking him in the eye. “[But] there was a little girl in California... she was bused to school every day. And that little girl was me.”
Mr Biden was visibly stunned. “I thought we were friends...” he said later. Observers wondered whether the pair would ever speak again. Nevertheless, Ms Harris has been in pole position to be Mr Biden’s running mate for some time.
When a photographer caught sight of Mr Biden’s handwritten speaking notes (below) at an event this week, the top entry was “Kamala Harris – do not hold grudges”.
Ms Harris would be the first black woman to become a vice-presidential nominee. She is the daughter of academics who immigrated to America. Her Jamaican-born father is a Stanford University economist, and her mother was a Tamil cancer research scientist.
After studying at the predominantly black Howard University, in Washington, she became a prosecutor, rising to be the first female, and first black, attorney general of California.
Ironically, her decades as a prosecutor may appeal to Republican voters; she has been accused by some Democrats of “keeping innocent people on death row”, despite her personal opposition to capital punishment.
She also once threatened to prosecute parents if their children did not go to school.
Ms Harris has been a rising star in the Democrat firmament for some time. In 2013, Barack Obama said of her: “She is brilliant and she is dedicated, and she is tough. She also happens to be, by far, the best looking attorney general in the country.”
The comment ignited a brief political firestorm in which the president was widely accused of sexism. Mr Obama called his friend to apologise. As a senator sitting on the senate committee investigating Russian election interference, she criticised Jeff Sessions, Donald Trump’s attorney general, live on television, and then targeted Brett Kavanaugh, his Supreme Court nominee.
Mr Trump called her “nasty”. Democrats called her the “female Obama”.
Ms Harris then entered the Democrat presidential race to great fanfare, on Martin Luther King Jr day, in January. However, her campaign failed to catch fire. After the initial ambush on Mr Biden, several debate performances fell flat, money dried up and poll numbers plummeted.
Ideologically, Ms Harris did not appeal to the more Left-wing activist base of the party that supported Bernie Sanders. Her centrist background – and scandal-free personal life, means she is the candidate Republicans may fear most. Ms Rice and Mr Biden worked closely together in the Obama administration. And Ms Bass is seen as a consensus candidate, who would not overshadow Mr Biden.
He will speak to each candidate individually next week. Whoever he picks will have their key campaign moment in the vice-presidential debate.
On Friday, Ms Harris rounded on those criticising her. Speaking at a virtual conference for young black women, Ms Harris said her entire career she had been told to “wait her turn”, but she “eats ‘no’ for breakfast”.
Urging listeners to embrace their “black girl magic”, she said: “There will be a resistance to your ambition. There will be people who say to you ‘You are out of your lane’.
“You know how many times I’ve been told it can’t be done? [That] nobody like you has done it before. I want you to be ambitious.”