Why California is Republicans' best bet to stop Trump

California is the biggest prize in the US presidential primary cycle – and, as Donald Trump, Ted Cruz and John Kasich will discover when they travel to the state later this month, the most complicated, demanding and expensive political battlefield.

For decades, the state has been irrelevant to presidential nomination contests.

With its primaries on the tail end of the election cycle, the last time the Golden State mattered to Democrats was in 1972, and California has not been relevant to Republicans since 1964.

This year is different. If Trump is to either win the 1,237 delegates needed secure the nomination or get close enough to effectively seal victory, then the finish line will be marked in California.

For Cruz, Kasich, and much of the Republican establishment, the contest is likely to be their last chance to stop the businessman. There are other important contests that will come first – particularly the one held in Indiana.

But none is expected to have a bigger impact on the outcome of the race than California. Trump hopes to perform well there in June off the back of his convincing victory in New York on Tuesday , which followed a string of losses.

Whatever happens, California’s 172 delegates don’t come cheap.

“California is a challenge to any candidate running,” said former state senator Jim Brulte, chairman of the state Republican party.“A statewide media buy is probably $2.5m to $2.8m a week.” Unlike New Hampshire or South Carolina, he added: “you can’t organize a state of 40 million people”.

The state is also a challenge because the Republican contest in California has a structure that bedevils pretty much any candidate who makes it to the end of the primary season.

For starters, only registered Republicans are allowed to vote. The more moderate members of California’s Grand Old Party have mostly peeled off and registered as “no party preference”.

Although they’ll be able to weigh in on the Democratic race, they can’t help someone like the ailing Kasich, the most centrist of the remaining trio of Republican contenders, all of whom will speak at the state party convention, just south of San Francisco, later this month.

California’s contest, like the one that has just taken place in New York, is less of a single primary than a slew of mini races.

There will be 53 separate winner-take-all races on 7 June – one for every congressional district. So it doesn’t matter how many Republicans there are in any one district because each is worth exactly three delegates.

That means that the most liberal districts, which have the lowest number of registered Republicans, become the most sought-after to Republican presidential candidates, who can claim victory by reaching just a small number of voters.

Republican US representative Tom McClintock’s central California district, for example, has 175,401 registered Republicans.

In contrast, Democratic US representative Nancy Pelosi’s San Francisco district has just 30,619 registered Republicans.

So how does a Republican win in a district like Pelosi’s?

“The honest answer is, ‘We don’t know’,” said Dan Schnur, director of the Jesse M Unruh Institute of Politics at USC. “There aren’t a lot of Republicans in that district, but there are some. All of a sudden their votes count a great deal.

“Logic suggests it will require a strong field organization like the one Cruz has put together,” he added. “Trump may start with a natural advantage in those communities, but it’s hard to tell whether he can put together a field organization to capitalize on that advantage in such short order.”

Cruz is organized in every congressional district. His state chairman is Ron Nehring, who spent four years as chairman of the California Republican party and ran for lieutenant governor. By the time the San Diego County Lincoln Reagan dinner rolled around on 26 March, the Cruz campaign had already begun meeting with delegates.

Kasich tapped tech entrepreneur Steve Poizner, a former state insurance commissioner, in February to lead his California campaign. Trump didn’t announce his Californian director, veteran GOP strategist Tim Clark, until 12 April – less than a month before absentee ballots are scheduled to hit the mail.

Still, Trump is leading in the polls in California and won decisively in Arizona, which has some demographic similarities with parts of southern California, and has also triumphed in liberal states such as Massachusetts.

“A lot of Republicans in California feel very passionately about undocumented immigration, his signature issue,” John J Pitney Jr, professor of American politics at Claremont McKenna College, said of Trump. “This goes all the way back to Pete Wilson and Proposition 187 in 1994.”

Wilson was governor then and was a major proponent of the measure which would have banned undocumented immigrants from using state services such as public education and non-emergency health care. It was approved by a hefty margin, 59% to 41%, but was later deemed unconstitutional.

Schnur predicts that Trump will “face widespread condemnation” in California, “but most of those people won’t be able to vote for him.” The USC Dornsife/Los Angeles Times Poll , he said, shows that “Trump support comes primarily from areas where Republicans have not traditionally done well, especially urban areas.”

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