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The most important primary of the presidential nominating contest just might be the very last one. On June 7, California could determine whether Donald Trump is able to amass enough delegates to clinch the nomination and avoid a chaotic open convention in Cleveland.

Now three veteran California Republican operatives are launching an effort to keep Trump from winning a mass of delegates in California, localizing the stop-Trump effort, by focusing on the fight, district-by-district.

Rob Stutzman, a campaign veteran, is leading the effort with two of his colleagues, Richard Temple and Ray McNally, and their goal is lofty.

“I think it’s possible Trump gets shut out in California,” Stutzman told NBC News.

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Trump must win 59 percent of overall remaining delegates to win the nomination – an increasingly difficult task in a three-person field with only 16 contests remaining. California is critical because of its huge prize - 172 delegates – or a gigantic 14 percent of the 1237 delegates needed for a candidate to win the nomination.

Sen. Ted Cruz, Trump's main challenger, is looking ahead to the state. He'll make two campaign stops there Monday.

Stutzman said that California’s system makes it more possible to defeat Trump by focusing very specific, targeted messages to Republicans in each district. Only 13 of California’s delegates are awarded to the winner of the state, the rest are doled out based on the winner of each Congressional district.

His focus on delegate allocation falls in line with the new focus of national stop-Trump movements, which have transitioned from focusing on state-wide wins to delegate-wins.

Related: Stop-Trump Group Has Found Trump's Weakness

Trump has failed to master the complicated minutia of delegate allocation, proving to be a major weakness of his candidacy. To correct and play catch-up, he has hired new staff and begun focusing on the process.

Related: Colorado Loses Reveal Chaotic, Overwhelmed Trump Campaign

The key in California, however, is that Cruz and Gov. John Kasich, the two other candidates remaining, don’t split the vote, handing Trump a victory.

“I wouldn’t be surprised if we had a website where you can enter your zip code,” telling their vote for Cruz would help Trump, or hit him.

Stutzman, who worked for former Governor Arnold Schwarzenegger and also successfully defeated Senator David Vitter’s effort to win the governorship last year in Louisiana, said he's at the fundraising stage of his efforts and plans to use his super PAC, Victory California. He’s recruiting local California donors to the effort while also communicating with the national donors who have been working to stop Trump to make sure their efforts are not duplicated.

He plans to ramp up efforts in the next ten days because early voting starts there on May 9.

A new poll released by Field shows that Trump is currently winning the state with 39 percent to Cruz’s 32 percent. Stutzman said Trump failed to garner more than 40 percent support among Republicans, and combined with California’s delegate allocation system, California could a challenging state for Trump.

California’s presidential primary arguably hasn't been a critical contest since 1972 when Sen. George McGovern won, giving him momentum to win the nomination. It hasn't mattered recently. By the time the state votes at the end of the process, nominees have been decided by previous states.