Lee, who has been called "the House's lefty conscience" will be California co-chair of Harris' presidential campaign.
"Watching Kamala's career in the East Bay and San Francisco for 20 years, I've witnessed her deep passion for justice and opportunity and I know she will be a president truly of the people, by the people, and for the people," Lee said in a statement obtained exclusively by CNN.
"She will increase working Americans' incomes, expand health insurance to more Americans and restore dignity and responsibility to the Oval Office. She is a leader uniquely qualified to bring us together and mobilize a movement of Americans to return power to the people."
For Harris, who is set to go to South Carolina this weekend
, it is an undeniable boost. While there is all sorts of data that suggests endorsements don't really move voters, for Harris, Lee's endorsement helps her burnish her progressive credentials and adds to her California show of force -- and 400 delegates will be up for grabs in Harris' home state on Super Tuesday.
A favorite of progressive outlets, Lee can amplify Harris' message, help her explain how she was a "progressive prosecutor"
-- an issue that's dogged her young campaign -- and essentially be a force multiplier. Lee is the fourth member of Congress from California to back Harris, though her fellow California senator, Dianne Feinstein, has said she intends to support Joe Biden, if he decides to run.
With her endorsement, Lee becomes the first CBC member to weigh in on the Democratic primary, a contest that for the first time includes two CBC members -- Harris and Sen. Cory Booker of New Jersey.
With 55 members, many in crucial southern states, the CBC will be closely watched as a barometer for African-American voters. Booker and Biden, should he run, will also be competing in the "CBC primary" as members weigh their relationships, their constituencies and candidate platforms as the primary grinds on.
Lee's endorsement might also foreshadow a show of force among other CBC members, potentially Rep. Maxine Waters and Karen Bass, two other progressive stalwarts from California.
Lee is a progressive icon, known for being the only member of Congress to vote against the authorization for the use of force in the days after 9/11 -- a stance called "prescient ... brave ... and heroic," 15 years later by The Intercept's Glenn Greenwald.
Not only that, she is against the death penalty and has tried to keep issues like climate change and poverty at the forefront of Democratic policies, even as the party took a more centrist approach in years past. Lee's bid for Democratic caucus chair was backed by liberal standard-bearers Alexandria Ocasio-Cortez of New York and Ro Khanna of California. Lee's loss to Hakeem Jeffries prompted an opinion piece in Essence Magazine called: "Barbara Lee Deserved Better."
Obviously, Harris and Lee have known each other for decades, and both have roots in Oakland. But, Lee's endorsement has never been an easy get. She broke with the CBC in 2008 Democratic primary, endorsing Barack Obama as other members backed Hillary Clinton.
In 2016, she held off on endorsing Clinton until the primary was over, again bucking the CBC and the party establishment and highlighting her progressive bonafides. She was instrumental in bridging the Bernie Sanders-Hillary Clinton divide, working with both campaigns to craft the 2016 unified party platform.
In 2018, Sanders campaigned for her in Berkeley, making this a potential blow for Sanders, who is likely to run again.
Lee's endorsement in full:
"When I was a young single mother attending Mills College, I invited Shirley Chisholm to come to speak on campus and was inspired to later work on her presidential campaign. I threw all my energy and support behind a visionary Black woman committed to making America work for all its people. Shirley set us on a path toward progress, and now Senator Kamala Harris picks up the baton. As just the third African American woman from a major party to run for President, I am so proud to endorse her candidacy as she continues this fight for equality, fairness, and dignity for all Americans. Watching Kamala's career in the East Bay and San Francisco for 20 years, I've witnessed her deep passion for justice and opportunity, and I know she will be a president truly of the people, by the people, and for the people. She will increase working Americans' incomes, expand health insurance to more Americans, and restore dignity and responsibility to the Oval Office. She is a leader uniquely qualified to bring us together and mobilize a movement of Americans to return power to the people."