Sebastopol residents Joseph Mastrolia and his children Domino, 13, left and Talulah, 11 cheer during a Bernie Sanders campaign rally Feb. 17 in Richmond. (Photo by Anne Wernikoff for CalMatters)

There’s a single number likely to be keeping Democratic candidates for president up at night, and it’s 15 percent.

Under the terms set by the national party, candidates can only win delegates — the partisan electors sent to the Democratic National Convention to secure the Democratic nomination — if they nab at least 15 percent of the popular vote. Those with vote totals under that all-important threshold get a grand total of zilch.

As candidates scramble to rack up support in the lead-up to the California primary on March 3, the latest polls here are likely to send all candidates not named “Bernie Sanders” scrambling especially hard.

A Public Policy Institute of California survey last week gave the Vermont senator at 32 percent of the likely Democratic vote statewide. No other candidate reached the 15 percent threshold — though it was within the margin of error for Joe Biden, Elizabeth Warren, Michael Bloomberg and Pete Buttigieg. And that was before Sander’s decisive victory in Nevada.

The good news for Sanders (and bad news for everyone else in the field) is reflected in the California current polling average from FiveThirtyEight. Sanders hovers near 30 percent, while Bloomberg, Biden, Warren and Buttigieg come in just under 15 percent.

If current trends hold, Sanders stands to win a huge share — if not an outright majority — of California’s 415 delegates (that doesn’t include “superdelegates” who aren’t elected and whose influence over the nomination process is more limited).

Of course polls aren’t static; they capture a moving target. Sanders is well-positioned, but here are more reasons he can’t count on a California delegate sweep:

  • Delegates aren’t all awarded based on the statewide vote total. Even if Sanders wins 30 percent of the Democratic ballots in California, two-thirds of state party’s elected delegates are doled out based on the primary results in each congressional district. That would give other candidates with geographically concentrated support in certain areas of the state a chance to tamp down Sanders’ apparent lead.
  • Though Californians are already voting by mail, most ballots have likely yet to be cast. A lot can and will happen between now and March 3, including a primary in South Carolina and a nationally televised debate.
  • Much of Sanders’ electoral good fortune can be attributed to the sheer size of the field, with moderate candidates like Biden, Buttigieg, Amy Klobouchar and now Bloomberg) dividing up the Democratic-but-not-Democratic-Socialist vote. A narrowing of that field would alter the math.

In 2017 California legislators voted to bump up the state’s presidential primary, pushing the election day from the late-season irrelevance of early June to join the ranks of the Super Tuesday states. The hope was that the largest state in the nation and the largest source of nominee-selecting delegates would actually make a difference in each party’s nomination process.

California may finally get its wish.

This much is clear: The biggest factor on March 3 will be California — which, along with Texas and 13 other states and territories voting that on that “Super Tuesday” — represent 40 percent of the U.S. population.

The first four primary or caucus contests in February are important not by dint of their delegate totals, but their timing and symbolism. The more pint-sized states, Iowa, New Hampshire, Nevada and South Carolina carry a total of 193 delegates. Wins there matter because they confer those coveted, if hard to quantify, markers of success like “momentum” and “electability.”

But California’s 415 delegates? That’s one serious haul. If Sanders wins big here, home to 9.1 million Democrats, it’s hard to imagine him not heading to the party’s nominating convention in July the presumptive nominee — or at the very least, the overwhelming frontrunner.

And so the state’s looming influence has prompted a new round of calls from party moderates for some candidates to gracefully take their leave from the race — before a Bernie victory is inevitable.

Even Andrew Yang, who recently made the natural transition from long-shot presidential candidate to CNN political commentator, called upon some of the remaining candidates to follow his lead for the sake of party unity. “Someone needs to pull an Andrew Yang,” he said.

For some, it’s a flashback to 2016, when Donald Trump benefited from a crowded field, a splintered vote and a dedicated base of supporters to win the GOP nomination despite regularly winning less than a third of the vote in many of the early states.

But so far, the field refuses to winnow itself. And March 3 approaches. In fact, for some voters it’s already here. According to a mail ballot tracker from Political Data Inc., nearly 750,000 Californians have already mailed in their Democratic ballots.

CalMatters.org is a nonprofit, nonpartisan media venture explaining California policies and politics.

Ben Christopher, CalMatters

CALmatters