They held their first joint appearance together in Wilmington, Delaware.
The announcement has energized Biden's campaign: More than $10 million has already been raised since yesterday.
Supporters say this new Biden-Harris ticket is a reflection of the diversity in America.
Senator Harris' mother is Indian and her father is Jamaican, making Harris the first woman of color to become a vice presidential nominee. A nod to the scores of Black women who for decades have been consistent and loyal supporters of the Democratic party.
Many know Harris after her presidential and have watched as she has ascended in her career.
In recent weeks, Biden has faced mounting pressure to choose a Black woman, especially as the community effectively saved his political career, and because of the racial uprisings.
Biden says this election is a battle for the soul of the nation and he says together the Biden-Harris ticket will address the climate, health care and economic crisis.
But the pair now face an uphill challenge against President Donald Trump, who has already gone on the attack.
The president and Republicans are calling Harris "phony" and saying she is too far left, especially when it comes to her stance on health care, gun rights and the environment.
Up until now, the presidential race has been pretty quiet. Expect that to change: There are less than three months until the election.
CBS2's John Dias got reaction from New Yorkers to Biden's pick for his vice president.
"We have a lot of work ahead of us, but I think they could get it done. I really do," said White Plains resident Denise Dockery.
One Harlem mother said this teaches her 4-year-old son he can shoot high.
"Just because you're Black doesn't mean you can't be anything that you want to be," said Yasmin Tillery. "The young black youth and the girls, I think it's a positive thing."
Harris was considered a front-runner when she launched her own bid for the White House, and even went after her former rival during a primary debate last summer.
"When you're debating against one another, you gotta say what you gotta say," said Highbridge resident Luis Santiago.
Harris was among 13 potential female candidates.
Trump tweeted out an attack ad targeting the new Democratic ticket. He also had sharp words for her record.
"She's very big into raising taxes. She wants to slash funds for our military... she did very poorly in the primaries, as you know," Trump said. "I was surprised that he picked her."
"He is not a rash politician. He thinks it through. He is very deliberate," St. Peter's University Prof. Alain Sanders said of Biden. "The drawbacks are, she might not ignite the Democratic base. She might not ignite part of the Democratic base which is known as the progressive coalition."
Sanders, a political science professor, said since Biden will be 78 on inauguration day, he needs to allay the fears of some voters who need to know the VP pick can step into the job.
"By indicating that he is willing to pass the baton to potentially a different kind of candidate than the traditional white male candidate, I think that has possibility of being very energizing," he said.
Within hours of the vice presidential announcement Tuesday the Biden campaign raised more than $10 million, CBS2's Aundrea Cline-Thomas reported.
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