Within days, Obama's former campaign manager David Plouffe and other high profile alumni from Obama's 2008 and 2012 election efforts joined her nascent campaign.
"He just wanted her to know that she was in a position to recruit anyone she thought they needed," said a source familiar with Obama's and Harris' conversation.
Democrats have coalesced around Harris as opinion polls swing in the party's favor, but a month ago some were openly questioning whether she was the right choice to beat Republican candidate Donald Trump when President Joe Biden was forced out of the race after a halting debate performance.
Obama waited to endorse Harris for several days, planning to stay above the fray in case there was a competitive nominating process. When it was clear no significant challengers were coming, he jumped in.
Ex-Obama advisers, including Plouffe and 2012 deputy campaign manager Stephanie Cutter, quickly joined Harris' campaign, cementing the former president's imprint on her political operation and underscoring his support for her and his continued influence within the party she now leads.
Obama, 63, the keynote speaker at the Democratic National Convention in Chicago on Tuesday, has known Harris, 59, for two decades, aides say. They first met at a California fundraiser in 2004.
Both rose in local politics, she as a prosecutor and he as a community organizer and state senator, before entering the U.S. Senate. Both have represented historical firsts - he as the first Black U.S. president, and she as the first woman and first Black and South Asian person to serve as vice president.
Harris was an early supporter of Obama's 2008 presidential campaign when she was San Francisco district attorney, showing up at his campaign launch in Springfield, Illinois, and knocking on doors for him ahead of the Iowa caucus that helped propel him to the nomination.
He admired her work in California, viewing her as tough, but created a stir in 2013 by calling her "by far the best-looking attorney general in the country" at a fundraiser they both attended.
He phoned her later to apologize.
Obama backed Biden's pick of Harris as vice president, according to a Democrat briefed on Biden's deliberations. He has kept in regular touch since the 2020 election, aides say. The former president and his wife Michelle have shared dinner with Harris and her husband, Doug Emhoff, at least a couple times.
"She was an early supporter of his and he was an early admirer of hers, without question," said David Axelrod, a longtime Obama adviser. "He will not be talking about someone he doesn't know."
UNSOLICITED ADVICE
Democrats' move not to mount a competitive nominating process after Biden stepped aside seemed to surprise the former president.
"Although you called for an open process and ... Democrats have ... put in place an open process, it appears that people feel very strongly that you need to be our nominee," Obama said in a call with Harris, according to a transcript released by his office.
A clip of the Obamas congratulating Harris warmly in the call was released by the Harris campaign in a one-minute video that has been viewed by millions.
A Harris aide said the former president now reaches out to Harris unsolicited with advice, which the aide said was welcome, even if she does not always take it. Harris spoke to Obama during her vice presidential selection process, the aide said.
"He has offered to support her campaign in any way that he can – including policy or strategic advice, fundraising, and of course campaign travel to help get-out-the-vote," another Obama aide said.
ECHOES OF 2008
Echoes of Obama have been evident at Harris' rallies since she became the Democratic standard bearer, drawing crowds of thousands.
"They're very different political figures, but certainly the excitement around her is familiar to those of us who were around in 2008," said Jamal Simmons, a former Harris adviser.
Obama's campaign buzzed with the themes of hope and change; Harris' campaign, with running mate Tim Walz, the governor of Minnesota, has found energy in promoting joy.
The Harris aide said she was pleased that both Obamas were speaking at the convention. Michelle Obama, who famously called on Democrats in 2016 to go "high" when Republicans went low with political digs, is hugely popular in the party and, polls show, would have had significant support for a presidential run herself had she shown a desire to pursue one. She didn't.
Meanwhile former President Obama remains an almost mythical figure among Democrats.
His eight-year presidency is viewed with nostalgia and his cultural status - he still releases closely watched music play lists and summer reading lists, seven years after leaving office - has kept him relevant with young people in the party's base.
BUMPS IN RELATIONSHIP WITH BIDEN
Obama's influence with Harris has grown at the same time that his relationship with Biden, his former vice president, has become more complicated.
Obama, along with former House Speaker Nancy Pelosi, is viewed by some as having been a driving force behind the party revolt that led Biden to step aside over his disastrous debate against Trump in late June.
Obama's aides say only that he served as a sounding board for the president.
A White House official acknowledged that Biden had been upset with those who wanted him to drop out of the race but said he was confident in his decision now and would work with Obama to make sure Trump is defeated in November.
Obama and Biden have had multiple ups and downs.
Some in Biden's orbit believe Obama put his thumb on the scale for Hillary Clinton to be the 2016 presidential nominee, rather than Biden, who was considering a run even while grieving the death of his son Beau. Obama gave a eulogy for Beau Biden.
"You can love and have affection for someone and it can still be complicated," said one Democrat with ties to both men.
Biden will not be present for Obama's prime-time convention speech. After making his own remarks on Monday night in Chicago, he is spending the rest of the week on vacation in California.
