Mike Bloomberg has 300 paid staffers on the ground in California. Tom Steyer has 100. Bernie Sanders has 90.

Here’s how unusual that is for a California primary campaign: Donald Trump had seven paid staffers in the state before the 2016 GOP primary. As in one more than six. He relied on 15,000 volunteers to win.

The best way for candidates to reach voters in the nation’s most populous state is still through TV and digital advertising. But there’s a big change this year that reflects California’s sudden importance in choosing the Democratic nominee: Candidates are spending big money to employ staffers to boost support leading up to the March 3 primary.

That could mean trouble for Minnesota Sen. Amy Klobuchar, even after her strong showing in Tuesday’s New Hampshire primary. Her campaign says it is “ramping up” operations in California but has not said whether it has any staffers in the state. She has spent no money on TV ads here.

Instead, Klobuchar is trying to build a campaign plane as she flies west.

“If you haven’t spent time building the (field) operation and don’t have the money to compete on TV, it’s going to be very difficult to compete,” said Democratic Assemblywoman Buffy Wicks of Oakland, who was the California field director for Barack Obama’s 2008 run and state director for Hillary Clinton’s 2016 primary campaign.

Obama had only 20 paid staffers in California before the February 2008 Super Tuesday primary. But Wicks had begun preparing the previous June. Eventually, she and the campaign trained 50 volunteer field organizers who worked 20 to 40 hours a week and established 40 field offices across the state.

“It was a full-on grassroots campaign,” Wicks said.

Even that wasn’t enough. Clinton won the California primary with 51% of the vote, to 43% for Obama.

Wicks had better luck in 2016 when she led the Clinton campaign in California. But she got a late start — just three months before the June primary — since the national campaign’s leaders didn’t think Clinton would be facing a challenger by the time the state voted. But Sanders proved resilient all the way through the primary season, and ultimately came within seven percentage points of beating Clinton in California.

The Clinton campaign had 30 staffers in California, but was able to tap into a large network of activists, including some Wicks had trained eight years earlier.

“The beauty of California, even if you don’t have a lot of time, is that we have a lot of volunteers across the state, a really highly trained activist base, who know how to do this,” Wicks said.

Sanders has been organizing in California since May, particularly in predominantly Latino communities. The Vermont senator’s campaign opened some of its 20 offices in more remote inland areas, “where folks haven’t been since the 1960s,” State Director Rafael Navar said this week at a Sacramento Press Club event recorded for The Chronicle’s “It’s All Political” podcast.

Bloomberg’s campaign has opened more than 20 offices in California since the billionaire former New York mayor entered the race in November, said State Director Chris Myers.

Other Democratic candidates have also been in the state for a while. Massachusetts Sen. Elizabeth Warren has nearly 50 paid staffers here, and former Vice President Joe Biden has 20.

Wicks said that trying to match other campaigns’ months of efforts now — a week after voting started in California and less than three weeks before election day — “is a very short runway.”

That’s the challenge facing Klobuchar, who spent the entire campaign polling in the single digits until this week and put all her efforts into making a strong early showing in Iowa and New Hampshire.

Pete Buttigieg, the former mayor of South Bend, Ind., who is now atop the Democratic field along with Sanders, faces a similar problem. He does have 15 paid staffers in California, but has yet to open a field office.

Still, Buttigieg has a footprint in California that Klobuchar lacks. He’s raised $12 million in California, far more than Klobuchar’s $2 million, and he just won the endorsement of the state’s second-ranking Democratic officeholder, Lt. Gov. Eleni Kounalakis.

Klobuchar, meanwhile, is relying on a loose network of supporters like Lisa and Steve Schatz. The Atherton couple have hosted or attended three fundraisers for Klobuchar since May and will attend another Sunday in San Francisco.

“She has a record of getting things done. She’s progressive but she’s practical,” Steve Schatz said.

“Everyone’s on social media — people are hearing about her,” Lisa Schatz said. “I can’t tell you how many people have contacted Steve (since the New Hampshire primary) and told him they want to make huge donations.”

Klobuchar raised $2 million nationally after Friday’s debate in New Hampshire and $2.5 million more after Tuesday’s primary. But she had only about $5 million cash on hand on Dec. 31, the end of the last fundraising period.

That stash would be gone in days in California, which is just one of 14 states holding contests on March 3. One-third of the delegates needed for the nomination will be allocated that day.

A candidate needs to spend at least $2 million a week for a month to run statewide on TV, said Tim Clark, who ran Trump’s 2016 campaign in California. Bloomberg has blanketed California with $33 million worth of TV ads, while Steyer, the other billionaire in the Democratic race, has spent nearly $16 million.

But it’s not enough to just air TV ads. Without a ground game, candidates will “struggle — especially in a multicandidate race like the Democrats have,” Clark said. “If your voters don’t see you on the ground, they may not show up because they don’t sense you have momentum and support.

“Maybe that’s where Bloomberg has it right,” Clark said. “If he has 300 staffers on the ground, he understands the dynamic that you can’t just buy it on the air. You have to show people that are willing to put their boots on the ground and say they are for you.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli