California and five other US states are holding presidential primary elections as part of the nominating process held in each state and territory.
California, the country’s most populous state, along with Montana, New Jersey, New Mexico, North Dakota and South Dakota are all voting on June 7.
The Associated Press said Monday that Hillary Clinton has already reached the 2,383 delegates needed to clinch the Democratic nomination.
However, party rival Bernie Sanders said it was too early to call the result, saying Clinton has yet to win the nomination.
The Real Clear Politics average poll in California, based on three surveys conducted in the state between May 26 and June 3, shows Clinton edging out Sanders, 47.7 to 45.7 percent.
The Republican primary is also taking place Tuesday, but the Democratic race is overshadowing it as Donald Trump, the presumptive Republican nominee for president, has no active opponents left in the primary race.