Mrs. Clinton does not need to win California to become her party’s nominee. She is expected to reach the 2,383 delegates needed to clinch the nomination roughly three hours before the California results are tallied, when the polls close in New Jersey, a mathematical fact that Mrs. Clinton’s allies have been reciting to reporters.

But losing the most populous state, the birthplace of political movements and trends that often shape the rest of the country, would deal Mrs. Clinton a blow and send her hobbling to the Democratic National Convention in July.

At a rally in Buena Park, Calif., on Wednesday, Mrs. Clinton implored Californians to vote. “You know that this primary in California on June 7th is really important,” she told the crowd. “California is where the future starts in America.”

A defeat would also embolden Mr. Sanders and his supporters, some of who have pledged unrest at the Philadelphia convention if their issues are not taken seriously.

“Every minute she has to spend on Sanders is a moment and a dollar she can’t spend on Trump,” said Steven Maviglio, a Sacramento-based political consultant. But, he added: “It’s the Super Bowl for Bernie. It’s game over. He has nowhere else to put his money when this is done.”

To that end, Mr. Sanders has already spent 18 days in the state, speaking to crowds of thousands. And his campaign has spent $1.5 million on an ad urging Californians to choose “a new direction for the Democratic Party.”