Los Angeles (CNN) When California moved its primary up to March, the decision was viewed as a boon for Sen. Kamala Harris. But now, with nearly two dozen Democratic candidates in the race, Harris is facing fierce competition on her home turf on a day so packed with critical primaries that it might as well be called the March 3 migraine.

The Golden State's move from June to Super Tuesday was intended to increase the political clout of a state that—despite having the world's 5th largest economy and the nation's most diverse population—is often relegated to being an ATM for the candidates.

Though the early state contests of Iowa, New Hampshire, Nevada and South Carolina will winnow the enormous Democratic field, California voters may wield extra heft in 2020 because vote-by-mail ballots will go out to many of the state's 8.6 million Democratic voters on Feb. 3, the same day that Iowans caucus.

Here in California -- where Harris has worked hard to make her own electability argument at recent events and rallies -- early polling and interviews with dozens of voters across the state suggest a remarkably fluid race.

How to allocate resources on that mighty Super Tuesday is a complex equation for Harris and all of the 2020 contenders, who will be trying to prove their appeal not only in California, but also in Texas, Minnesota, and southern states like Alabama, North Carolina and Tennessee (where Harris, among others, is trying to consolidate black voters behind her bid for the White House).

Though Harris easily won her 2016 election to the Senate and remains popular here, her rivals clearly see an opportunity for an upset as more than a dozen of them head to the Golden State this week, pairing appearances at the California Democratic Convention in San Francisco with fundraisers and other campaign events.

Minnesota Sen. Amy Klobuchar was the latest to set her mark on California Tuesday with two events in the Los Angeles area and an appearance on "Jimmy Kimmel Live!" She led her remarks in Santa Monica by praising Harris as a "fantastic" candidate, and adding the sure-bet applause line: "I like to say: Let the best woman win." But in deep-blue California, she noted that Democrats have "had a little trouble in the heartland."

"The number one thing I hear out there, by the way, is we have to win. We have to beat Donald Trump," the Minnesota senator told her audience, listing the central tools in her arsenal: humor, staying focused on an "optimistic economic agenda," and not going "down every rabbit hole with him."

"A lot of this is going to be pushing back at him in new ways, making the case that he made these promises to people, especially in the Midwest, and he hasn't come through on that," Klobuchar said.

Unapologetically reeling back the liberal agenda of some of the Californians in the audience, Klobuchar called at one point for questions that weren't about impeachment: "I'm happy to answer those questions, but we've got to remember what everyone's thinking about in Nebraska. Right? OK," she said -- before calling on a questioner who she said looked "so Nebraska."

A fluid race

Because of the size of the field, the deep ties of other top contenders and California's complex rules for awarding most delegates by congressional district (only if a candidate crosses a 15% threshold), Harris may not eke out much of a home state edge.

But even in a split decision, strategists here say that Harris could glean enough California delegates to vault herself into the top tier after the first four contests -- giving her enough staying power to outlast many other candidates in the race.

Harris ranked third among the candidates with 17% in April's Quinnipiac poll of California Democrats. Former Vice President Joe Biden led Democrats and Democratic leaners with 26%, followed by Vermont Sen. Bernie Sanders at 18%. (That's higher than where she falls nationally; she's at 7.4% in the Real Clear Politics average of polling ).

Still, many voters told CNN in interviews that they are still deciding among at least three or four candidates. Harris, South Bend, Indiana, Mayor Pete Buttigieg, Massachusetts Sen. Elizabeth Warren, former Texas congressman Beto O'Rourke and Sanders are the names most often mentioned when voters list their top tier.

Sanders crisscrossed California in 2016, holding 27 campaign events in the month before his June primary matchup with Hillary Clinton. His die-hard supporters welcomed Sanders back when returned to campaign for congressional candidates in 2018.

Buttigieg, who recently headlined an LA fundraiser co-hosted by actors Gwyneth Paltrow and Bradley Whitford, is a favorite of Democratic activists who hope his second quarter fundraising numbers will vault him into the top tier. O'Rourke recently wrapped a four-day swing by telling San Diego voters that he felt at home in the Golden State.

Biden established strong ties within this state over his decades in politics, and that has bolstered his standing here both among average voters as well as big dollar bundlers. He demonstrated his Hollywood appeal by drawing a slate of heavyweight industry players to his recent LA fundraisers, including media mogul Peter Chernin, producer Jeffrey Katzenberg, ICM Partners' Chris Silbermann and former Google CEO Eric Schmidt.

During a quick stop for tacos with Los Angeles Mayor Eric Garcetti, the former vice president also hinted at California's importance in his own path to the nomination.

When asked whether he was worried about the elongated primary process, Biden predicted the race would wrap up more swiftly than expected. Pointing to California's trove of delegates, he said that the field "is going to be winnowed out pretty quickly, here in California as well."

"In order to get any delegates in a congressional district, you've got to get 15% of the vote" in California, the former vice president told reporters during his recent lunch stop at King Taco #10 in Los Angeles. "So, this is going to work its way through relatively quickly for all of us."

When asked by a reporter whether California would decide the race, Biden demurred.

Despite Californians' familiarity with Biden, he most often comes up in interviews with voters as an afterthought -- a sort of fail-safe option who is viewed as a solid pick against President Trump.

Ted Vaill, a filmmaker who sits on the board of both the Malibu and Pacific Palisades Democratic Clubs, was among many voters who said he would vote for Biden, but believes that the race right now is still anyone's to win.

Vaill supported Sanders last cycle, but is concerned that both Sanders and Biden have a "hell of a lot of baggage." During an interview before Klobuchar's event Tuesday in Santa Monica, he said he was worried that Republicans assembled a massive opposition file on "Crazy Bernie" that was not deployed in 2016, because Hillary Clinton became the Democratic nominee.

He listed his top choices as Klobuchar, Harris, Warren and Sanders. Buttigieg "intrigues" him, he said, potentially as a vice presidential candidate. Vaill also gave money to Montana Gov. Steve Bullock because he would like to see him on the debate stage. He said he was drawn to Klobuchar and Harris, both former prosecutors, because of their toughness — a trait he believes will be useful in the ultimate matchup against Trump.

Harris "is just going to clean (Trump) up if it happens—with the comebacks," Vaill said, noting the former California attorney general's prosecutor credentials. "What's he going to say about her? Does he even have a name for her?"

Reminded of Trump's recent remark that Harris was "nasty," Vaill quipped: "Is that bad?"

Cracking California

Harris has been back to campaign in California a great deal already this year, raising money, organizing voters and carefully assembling a strong coalition of California's progressive leaders to help bolster her fight, including California Gov. Gavin Newsom.

In a show of strength before the California Democratic Convention this weekend, Harris released a list of 33 Democratic State Assembly members who are backing her campaign -- a number that represents more than half the Assembly's Democratic caucus.

Those members join a long list of other elected officials who have endorsed her, including members of the House of Representatives -- Barbara Lee, Ted Lieu, Katie Hill, Julia Brownley, Nanette Barragan -- and key players like San Francisco Mayor London Breed, Sacramento Mayor Darrell Steinberg and Long Beach Mayor Robert Garcia.

As Harris did in her Senate race against Democratic Congresswoman Loretta Sanchez — a same-party opponent who she drew because of California's "top-two" primary system — she is working to prove her viability both by courting donors and by refusing to cede any demographic voter groups

In 2016, she made a strong push to win Latinos, even though pundits predicted many of those voters would support her opponent. Her co-chairs were two hard-hitting political players: former California Assembly speaker John Perez and Latino civil rights icon Dolores Huerta. In 2020, Huerta is co-chairing Harris' presidential campaign with California Rep. Lee.

"She has a name recognition advantage in this state and ten years of a track record that Californians have watched — for San Franciscans, even longer than that," said Santa Monica Democratic Club Chair Jon Katz, noting Harris' stints as attorney general and district attorney of San Francisco. "The people who are progressive, but are also elected in California, they are all getting behind her. I think that will help drive her in this state."

In mid-May on Mother's Day Eve, in the midst of a five-event sprint across the state, Harris arrived at the recently-founded women's club known as The Jane Club in central Los Angeles.

Before a receptive audience who crowded around her in a room filled with tea lights and bud vases, Harris applauded the "badass" women who had assembled to see her despite all the things they were juggling.

As a call to action for her 2020 campaign, the California senator recalled her "bittersweet" 2016 victory party in Los Angeles on the same night that Clinton lost.

"We all woke up the next morning — Woke," she said to laughter. "We might have been sleeping the night before, but not the next day! We all figured out we can't take anything for granted."

One of Harris' questioners told her she only had two hours a week to devote to political activism. "How would you direct me" to spend that time, she asked Harris.

Harris encouraged her listeners to invest their time into turning out voters — either by helping local groups who are registering new voters in California or assisting groups that are trying to win back the Senate, all while focusing on seats "that we can actually turn."

"Because I'm at this point all about winning," Harris concluded to laughter as the room burst into applause.