By Eric Bradner, CNN

(CNN) -- California Sen. Kamala Harris talked in deeply personal terms about her relationship with Joe Biden's late son Beau during their first appearance Wednesday as the Democratic 2020 ticket.

Biden, the presumptive Democratic nominee, said in a speech in Delaware he'd first learned about Harris through her relationship with Beau Biden, who died of brain cancer in 2015.

"Kamala, you've been an honorary Biden for quite some time," he said as he introduced Harris, who he selected as his running mate Tuesday.

Harris said that during the Great Recession, she as California attorney general and Beau Biden as Delaware attorney general spoke on the phone daily -- sometimes multiple times per day.

"I learned quickly that Beau was the kind of guy who inspires people to be a better version of themselves. He really was the best of us. And I would ask him, where'd you get that, where'd this come from, he'd always talk about his dad," she said. "The love that they shared was incredible to watch. It was the most beautiful display between a father and a son."

Before Harris spoke, Biden touted her status as a history-making vice presidential nominee in their first appearance together Wednesday in Delaware.

Biden said that with Harris now on the Democratic ticket, "Little Black and Brown girls, who so often feel overlooked and undervalued in their communities, but today, just maybe they're seeing themselves for the first time in a new way."

Biden began his speech in Wilmington by thanking other women his campaign vetted for the vice presidential pick, before he chose Harris -- the daughter of Indian and Jamaican immigrants -- on Tuesday. She is the first Black woman and first person of Indian descent on a major party's presidential ticket.

"As a child of immigrants, she knows personally how immigrant families enrich our country," he said. "Her story is America's story."

Biden also lambasted President Donald Trump for immediately referring Tuesday to Harris as "nasty," a term Trump has directed at other women in the past.

"Is anyone surprised Donald Trump has a problem with a strong woman, or strong women across the board?" Biden said.

He added: "Kamala Harris has had your back and now, we have to have her back. She's going to stand with me in this campaign, and all of us are going to stand up for her."

The speech comes the day after Biden chose Harris after a months-long vice presidential vetting process, making her the first Black woman and first person of Indian descent to be tapped for a major political party's ticket.

Their spouses, Jill Biden and Doug Emhoff, will also attend the speech in Wilmington, Jill Biden spokesperson Michael LaRosa said.

In a tweet that could preview their message, Biden tweeted Wednesday morning that if he and Harris win, "we're going to inherit multiple crises, a nation divided, and a world in disarray. We won't have a minute to waste."

"That's exactly why I picked her: She's ready to lead on day one," he tweeted.

Harris on Wednesday tweeted a biographical video that started with Biden asking her to join the ticket and echoed Biden's core message.

"We are in a battle for the soul of this nation. But together, it's a battle we can win," Harris tweeted.

After the speech, Biden and Harris are set to appear together at an online fundraiser for "grassroots" small-dollar donors.

Whether and how the two can hit the campaign trail, together or separately, remains to be seen. Next week's Democratic National Convention would ordinarily kick off a frenetic two-and-a-half month sprint to the November 3 election, but the coronavirus pandemic has effectively sidelined Biden from campaigning. He spent July delivering weekly speeches detailing planks of his economic agenda.

The last time Biden held a campaign rally -- March 9 in Detroit, on the eve of the Michigan primary -- Harris appeared with him on stage, as the 77-year-old former vice president called himself a "bridge" to a new generation of Democratic leaders.