Bernie Sanders’ latest California rally was thrown together in less than 24 hours, after bad weather forced him to cancel a planned campaign trip to Wyoming. But the event at the Wiltern theatre in Los Angeles was nonetheless a sell-out, with an overflow line stretching all the way down the block outside.

The Wiltern is more accustomed to hosting bands than presidential candidates, but the Vermont Senator received a rock star’s welcome. “We can argue about whether Vermont is more progressive than California,” he said as the cheers died down. “But I feel very comfortable here tonight.” Which is lucky, he might as well have added, since he’ll soon be spending a lot of time in the Golden State.

As the overwhelming winner of this week’s caucuses in Utah and Idaho, Mr Sanders nibbled ever so slightly into Hillary Clinton’s substantial delegate lead in the Democratic race, despite her victory in Arizona. But he has spent the past two days even further west, with rallies in San Diego and LA, where he also appeared on Jimmy Kimmel’s talk show and YouTube political show The Young Turks.

California Democrats don’t go to the polls until the last big day of the primary season on 7 June, but Mr Sanders has said he intends to remain in the race as far as the finish line. Given Ms Clinton’s current lead, he would have to enjoy several big wins in several big states to have a shot at overtaking the front-runner. There is none bigger than California.

The Sanders campaign plans to open offices across the state in the coming fortnight, and Mr Sanders is reportedly counting on his allies from California’s labour unions and environmental organisations to keep bringing large crowds to his rallies here. "The road to the White House goes right through the West," he told the crowd at the Wiltern, "and right through California".

Ms Clinton has also been in California this week, for a series of fundraisers in the Bay Area and Los Angeles. But as if to highlight the differences between the two campaigns, entry to Mr Sanders’s LA rally was free, while the cheapest tickets to Ms Clinton’s celebrity-studded event at the Avalon nightclub in Hollywood on Thursday cost $500 (£355).

The most populous state in the US has 546 Democratic delegates, almost twice as many as its nearest competitor, New York. During the 2008 primaries, with Barack Obama dominant, Ms Clinton carried California against the run of play, thanks in large part to the longstanding support she still enjoys among Latino voters.

Eight years ago, her win came too late to affect the outcome of the nomination. Another victory in June, however, would see her clear to the convention. She is more than a dozen points ahead of Mr Sanders on average in recent California polls, although he has already overcome similar odds in other states during this year’s contest.

If he continues to defy the arithmetic and sets up a final showdown with the former secretary of state here, it would be the first time the California primary had such a major role to play since anti-war liberal George McGovern clinched the Democratic nomination in 1972. He went on to lose to Richard Nixon in a general election landslide.

For Republicans, California also promises to be uncharacteristically significant in 2016. The state, which is worth a crucial 172 delegates to the GOP candidates – by far the most of any state in the Republican race – will probably be the party establishment’s last chance to deprive Donald Trump of the delegate majority he needs to secure the nomination ahead of the convention in July.

Though her parents are long-time Clinton supporters, student Melissa Avina, 19, was so converted to the Sanders cause that she went canvassing for him in Arizona. But if Ms Clinton is the nominee, Ms Avina said she and her friends would still vote for her. "It’s that or Donald Trump," she said. "As Latinos, he makes us afraid for ourselves and for our families. Hillary is the lesser of two evils."

Mr Sanders points out that polls of potential general election match-ups put him further ahead of the GOP candidates than Ms Clinton. And not everyone at the LA rally shared Ms Avina’s pragmatism. Margie Hoyt, 57, said her Democratic club had endorsed Ms Clinton, but that she would still vote for Mr Sanders as a write-in if he were not the nominee.