But they are not a surprise.
While Obama occasionally wades into state-level politics, Harris and the president are longtime friends. In 2013, Obama apologized to her after telling a group of wealthy donors in Silicon Valley that she is the "best-looking attorney general."
Biden said the Senate "needs people like her — leaders who will always fight to make a difference and who never forget where they come from."
Sanchez's campaign did not immediately respond to requests for comment.
Obama is popular in the Democratic-leaning state, and his involvement could provide a boost for Harris in a race that represents a historic first in California — two minority women, both Democrats, in a runoff to replace retiring Sen. Barbara Boxer.
The matchup marks the first time since voters started electing senators a century ago that Republicans will be absent from California's general election ballot for the Senate. Under California election rules, only two candidates — the top vote-getters — advance to the November election, regardless of party affiliation.
Harris entered the race last year and quickly established herself as the front-runner for the seat, leading in fundraising and polls. She has been endorsed by the state Democratic Party and has a long list of prominent supporters, including Gov. Jerry Brown and Massachusetts Democratic Sen. Elizabeth Warren.
If elected this fall, Harris, the daughter of immigrants from India and Jamaica, would set historical marks. She would become the first Indian woman to hold a Senate seat and the second black woman elected to the Senate. Illinois Sen. Carol Moseley Braun was elected in 1992 and served one term.
Sanchez, if elected, could become one of the first Latinas to hold a U.S. Senate seat. Catherine Cortez Masto, who is also Hispanic, is the Democratic candidate for outgoing Sen. Harry Reid's seat in Nevada.
As fellow Democrats, Harris and Sanchez hold similar positions on many issues, including abortion rights and immigration reform.
