She held off a surprisingly strong challenge from Vermont Sen. Bernie Sanders.
White House officials say Obama's announcement could come within days, although not before Democrats in New Jersey, California and four other states vote Tuesday.
"Hillary Clinton wants small, incremental changes. We want to transform this nation," Sanders said.
Hillary Clinton arremetió contra Donald Trump por hablar en "formas llenas de odio, muy prejuiciosas y realmente inaceptables"
Clinton kicked off her California campaigning at Los Angeles Mission College in Sylmar.
The U.S. Democratic presidential candidate lambasted Donald Trump's foreign policy platform as "dangerously incoherent" in a speech on Thursday.
She is continuing her efforts to draw contrasts with Donald Trump by stressing her support for immigration reform in a California town close to the Mexico border.
La ex primera dama y precandidata demócrata dijo en un discurso en San Diego que el aspirante republicano es susceptible, irracional y no está preparado para ser presidente
Encuesta confirma impopularidad de ambos aspirantes entre los electores
Según una encuesta divulgada por la Universidad Quinnipiac, 45% de los electores apoyan a Hillary Clinton, Donald Trump sería respaldado por 41% del electorado