California could be Bernie Sanders' last stand, but losing to him would mean a limp across the finish line for Hillary Clinton.
Amanda Walker US Correspondent
It's eight years to the day that Hillary Clinton withdrew from the race against then Senator Barack Obama.
She's hoping her current rival will follow suit now that the latest Associated Press count indicates she has reached the necessary number of delegates to clinch the Democratic nomination.
Dogged to the last, Bernie Sanders has other ideas.
On Monday, raspy-voiced after months on the campaign trail, he insisted he will not back down.
Bernie Sanders Campaigns In California Ahead Of State Primary
Bernie Sanders has won 20 states and more than 10 million votes
His hope is that a good performance in tomorrow's primaries could sway superdelegates, who are free to support any candidate.
Because superdelegates can change their minds up until the party's convention in July, Mr Sanders argues that Mrs Clinton will not officially have clinched anything.
U.S. Democratic presidential candidate Hillary Clinton attends a panel on healthcare in San Juan Play video "Who is Hillary Clinton?"
He said: "When we started this campaign a little over a year ago in Burlington, Vermont I think people ... thought this campaign would not go very far."
They did and they were wrong.
While the noise of the Trump show has drowned out most things in its path, the Sanders campaign has maintained its ability to astonish - enthusing young voters and consistently drawing huge crowds.
He's won 20 states and more than 10 million votes.
The senator from Vermont has proved himself a force to be reckoned with against a formidable opponent.
But California could be his last stand. It's America's largest and one of its most progressive states. It really matters for Mrs Clinton.
The polls put her in a virtual tie against Mr Sanders with three recent polls all showing her leading by just 2%.
Losing to Mr Sanders would mean an embarrassing limp across the finish line after a hard-fought race.
If she wins it sets the stage for a stronger fight against presumptive Republican nominee Donald Trump.
For Mr Sanders it leaves a tough dilemma - stay in or quit the race.
Mrs Clinton has the experience, money and support that should have made this look easy.
In California, Hollywood royalty including George Clooney, Tom Hanks, Leonardo DiCaprio and Ben Affleck are rallying around her.
But Mr Sanders is also in good company - Susan Sarandon is a vehement follower and Steven Spielberg is also "Feeling the Bern".
Mr Sanders insist he's the candidate who can win against Mr Trump in November's election.
There are numerous opinion polls that back that theory.
His current game plan would see neither him nor Mrs Clinton with enough pledged delegates, making July's convention a contested one.
An anti-establishment candidate getting party faithful superdelegates to switch sides is a long shot.
But Mr Sanders seems intent on giving it a go
Sky News