Vermont Sen. Bernie Sanders’ enthusiastic legion of young, independent and progressive supporters turned out to be way better at jamming rallies and concerts than they were at actually voting.

While hordes of Californians were eager to “feel the Bern” at more than 30 high-energy — and even higher-visibility — events across the state over the past couple weeks, former Secretary of State Hillary Clinton steamrolled Sanders in a surprisingly easy double-digit victory in the state’s Democratic presidential primary Tuesday.

That margin likely will shrink when the well over 1 million still uncounted California votes are tallied, but the final result will be nothing like the tight, “flip-a-coin” result many pollsters had predicted.

“The young (voters), the poor ones and those with no party preference were Bernie’s key supporters ... and they didn’t show up,” said Paul Mitchell, vice president of Political Data Inc., who studied more than 21,000 voters who had mailed in their ballots. Those who did turn out were older, wealthier and more likely to be registered Democrats — exactly the voters Clinton was targeting.

What was most surprising is that while 25 percent of California’s record surge of more than 2 million new registrations was made up of voters aged 35 years and under, they made up only 10 percent of those who had mailed their ballots back by the Monday before election day.

‘So why not vote?’

“These are people who had just registered, so why not vote?” Mitchell asked.

The complicated process independent voters had to go through to cast a ballot in the Democratic primary also did Sanders no favors. While independents could vote for Clinton or Sanders, they had to ask specifically for a Democratic ballot. No request, no ballot.

That was especially bad news for the many no-party-preference voters who cast their ballots by mail. Only about 15 percent of them asked for the Democrat ballot, which meant they were likely shut out in the presidential voting.

The extra effort needed “likely frustrated a lot of (independent) voters” who might otherwise have voted for Sanders, Mitchell said.

The polls last week showing Clinton’s shrinking lead also played a major role in the final numbers. After a series of surveys showed the California race had become a toss-up, Clinton scrapped plans to spend more time campaigning in New Jersey and flew to California for the final days before the primary.

Clinton “has a long history in California and really wanted to win the state,” said Buffy Wicks, the Clinton campaign’s state director. California was also the last state primary, and a Clinton win in the nation’s biggest state would make it hard for Sanders to argue that he was the “people’s choice” in the Democratic race.

Clintons blitzed state

Clinton and her husband, former President Bill Clinton, “did 43 events in the last five days,” Wicks said. “They were crisscrossing the state and showing people we were fighting for every vote.”

But Sanders was also campaigning at a frenetic pace, hitting cities large and small in a two-week-long schedule that finished Monday with a concert and rally that drew an estimated 10,000 people to the Presidio’s Crissy Field.

While Sanders looked like the candidate with the momentum, Clinton’s team was working behind the scenes to get their people to the polls. Drawing on their experience from 2008, when Clinton used a late run to beat Barack Obama in California’s presidential primary, Clinton’s supporters ran multilingual phone banks, cajoling likely voters to cast their ballots.

“We told people to take that ballot off the kitchen cabinet, fill it out and put it in the mail,” Wicks said.

The result was seen on election night. Clinton broke on top when the first mail ballots were reported soon after 8 p.m. and never gave up the lead.

“Clinton jumped to a huge lead, larger than expected, in the first mail results,” said Mark DiCamillo, director of the Field Poll, which last week showed the former New York senator with a 45 percent to 43 percent lead over Sanders.

The mail ballot margin was about 400,000 votes, which held through much of the night.

“Clinton and Sanders virtually split the votes at the polls on election day,” DiCamillo said. But the Clinton team “is very good at corralling votes, at getting their votes in the bank early.”

The tide began to turn last weekend, Wicks said.

“We felt a huge surge in momentum,” she said. “We had 22 percent more volunteers than we scheduled show up for our phone push. We had 5,500 volunteers make 2.2 million calls in the last few days.”

Backers not discouraged

Clinton’s win didn’t discourage Sanders or his backers.

At a Sanders event in San Francisco Tuesday, Fred Werner, 43, wore a Bernie yarmulke as he spoke glowingly of the senator’s decidedly non-concession speech.

“Bernie Sanders himself has always said — he even said it in his speech — this is not about Bernie,” Werner said. “The key thing is keeping this movement mobilized to actually change the way politics are happening in America. ... He is saying we’re not giving up, we’re fighting on.”

But while Sanders has vowed to continue his battle until the Democratic National Convention in July, he has reportedly laid off half his campaign staff and has returned to his home in Vermont to consider his options. He has a meeting scheduled with President Obama Thursday, where they will likely talk of the future of his campaign.

Chronicle staff writer Jenna Lyons contributed to this story.

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com Twitter: jfwildermuth