Democratic candidates Hillary Clinton and Bernie Sanders are in a dead heat as they head into the last stretch of the United States presidential primary in the most delegate-rich state of all - California.

Even though Mrs Clinton needs fewer than 70 delegates to bag the nomination, neither candidate seems to be backing down for fear that a loss in the Golden State would signal weakness in their respective campaigns.

Mrs Clinton changed her campaign schedule and has been working the ground in California, instead of New Jersey, which also goes to the polls tomorrow along with four other states: Montana, New Mexico, North Dakota and South Dakota.

Between her and her husband, former US president Bill Clinton, the couple have held around 30 campaign events in the past five days, hoping to eke out a win in California, which has 546 delegates up for grabs and was supposed to be an easy win for Mrs Clinton.

In 2008, when she was up against Mr Barack Obama, she won the state by 8 percentage points.

Noting the late shift in Mrs Clinton's plans, Mr Sanders told reporters: "I wonder why (Mrs) Clinton and her husband, Bill, are back in California... I thought we had lost it; it was all over. But I guess (she) maybe is looking at some polling (that) would suggest otherwise."

The two candidates were neck and neck in the most recent NBC/Wall Street Journal/Marist poll released last week, with Mr Sanders holding a 2 percentage point lead over Mrs Clinton, which is within the margin of error.

Not leaving anything to chance, both candidates have spent millions of dollars on television advertisements, hoping to energise their respective bases to get out and vote.

According to media reports, Mrs Clinton has ads in Spanish, Mandarin, Vietnamese and other languages to reach out to minority voters.

"She has been spending her money in the state on the ground game, focusing on turning out her base rather than persuading Sanders voters to move her way. This gives her the best shot at winning without alienating other Democrats," said political science professor Thad Kousser from the University of California, San Diego.

But a surge in new registrants - more than 1.5 million from January to the middle of last month, according to political consultants Political Data - is good news for the Sanders campaign.

Many of these new registrants would include young, first-time voters, many of whom have shown a preference for Mr Sanders.

But the question is whether they will eventually turn out to vote, said Dr Kousser.

Despite the emphasis on California, it is likely that Mrs Clinton will reach the 2,383 delegates needed to clinch the Democratic nomination even before the polls close in California, by picking up delegates in New Jersey, which is on the east coast of the country.

"I don't think that two months from now, it will make much of a difference if she does a little bit better or a little bit worse on Tuesday," said Professor Richard Lau, chair of the political science department at Rutgers University in New Jersey, adding that it is impossible for her to lose California in the general election.

But in the shorter term, after a long and bitter primary race, some feel winning a Democratic stronghold is exactly what the Clinton camp needs.

Dr Kousser said: "If Hillary Clinton loses, she limps into November, and the only story journalists have to write for the next three months is about why she is running such a poor campaign.

"If she finishes a few points higher and wins, she gets to take a victory lap and the narrative is about what a great campaigner she is," he added.