At a Trump rally in San Diego, two lines of people faced each other, separated by the width of a police officer's bicycle. One side was composed mostly of young Latinos, many of them waving Mexican flags.

"F@#% Donald Trump!" they shouted. "F@#% Donald Trump!"

The other side was mostly made up of young white men, many wearing or waving American flags.

"Build! The! Wall!" they shouted. "Build! The! Wall!"

I think he's going to get his clock cleaned in the general election because he doesn't appeal to those new groups that our party needs - Mike Murphy, Republican strategist

This was supposed to be the year the Republican party increased its share of the Latino vote by just enough — only a few percentage points — that it could ride that support all the way to the White House.

But Donald Trump's candidacy is undoing years of work wooing Latino voters.

The San Diego rally attracted about 1,000 protesters and, like several rallies before and since, pitted a largely white Trump-supporting audience against a largely Latino group of anti-Trump protesters. The violence was bipartisan.

Socked in the nose

"Is my nose broken?" Jack Schwarz asked, his face smeared with blood, his nose is swollen and turning black at the bridge during the rally.

Jack Schwarz says he was punched in the nose by a Trump supporter, who was led away in handcuffs by police. (Kim Brunhuber/CBC) Schwarz said a Trump supporter "socked him in the nose." Shortly after, a man was led away in handcuffs by police.

Close to one thousand people demonstrated against Trump outside his rally in San Diego (Kim Brunhuber/CBC) This is what happens when the Republican party's presumptive nominee comes to town.

Party insiders watched Trump supporters like John Puhek incite the very people they were trying to attract. He came to the San Diego rally wearing a T-shirt with "Trump El Jefe" printed on it, and a sombrero that read "Make America Great Again."

"It's not about race, it's about culture," Puhek said. "And that's what Trump's about."

It's about party culture

Heading into this election, Republican operatives like Sacramento's Mike Madrid knew the party's culture had to change. Its appeal to white voters had maxed out.

Donald Trump's negatives among Latinos have reach an "unprecedented" 90 per cent. (Kim Brunhuber/CBC) "It's necessary for a couple of reasons," Madrid said. "The first is that the Latino vote, both in terms of the population, as well as the voter base, is the fastest-growing segment of the national electorate."

Every year, close to one million more Latinos become eligible to vote. According to the Pew Hispanic Center, they're expected to account for nearly half of the growth in the eligible electorate between now and 2030.

"Simple mathematics suggests that it's not going to be very long before the party has no more growth opportunities," Madrid said.

Source of new voters

San Diego resident Chuck Tyson says he no longer supports the Republican Party because of its increasing intolerance under Trump. (Kim Brunhuber/CBC) Madrid specializes in Latino voting trends, and has worked tirelessly for decades to convince California's Hispanic community that Republicans shared their values.

"It's the battleground states where Latino voters are starting to have a greater impact," he said. "States like Arizona, Colorado, Nevada, even Virginia, North Carolina, have significantly increasing numbers of Latino voters, which are increasingly voting Democratic and taking what were battleground states out of play."

And then came Trump.

Unprecedented levels of negativity towards the candidate - Mike Madrid, Republican operative in Sacramento

"Spending 20 years working to build up the Republican party with Latinos has been challenging and difficult. And just as we were beginning to see some successes, to have that all kind of come crashing down..."

Ask Latinos what they think of Donald Trump and polls suggest nine out of 10 will say something negative.

"Extraordinarily high numbers," Madrid said. "Unprecedented levels of negativity towards the candidate."

California-based Republican strategist Mike Murphy, who has come out against his party's own candidate, believes there can only be one outcome in the general election.

"I think he's going to get his clock cleaned in the general election because he doesn't appeal to those new groups that our party needs," Murphy said.

White voters unhappy with racism

Many Republicans realize they can't recapture the White House without getting a significantly greater share of the Latino vote. Even some white Republicans are starting to turn.

Andrea Paul, 29, a born-again Christian and a Sunday school teacher, was practically in tears as she confronted an outspokenly anti-Mexican Trump supporter holding a megaphone and a Bible outside the San Diego Trump Rally.

"My entire family is still Republican as hell!" she said and, until a month ago, so was she.

"I have never voted Democrat in my life," Paul said. "This will be the first year that I do because Donald is just a spectacle."

Protesters stayed long after the speech was over, and some Trump supporters stayed to confront them. At first police tried to separate the groups with their bicycles as barriers. Riot police came later. (Kim Brunhuber/CBC) Not far away, Chuck Tyson held up a home-made sign that read 'What happened to my father's Republican party?'

"My father was a Republican and he raised us all to be Republican, and it's a shadow of what it was before," Tyson said. "And the intolerance, it has just become unbearable. And that's why so many people have left the Republican party... myself included. This is not the type of people he raised us to be."

When asked what it would take to fix the Republican party, Tyson sighed.

"I don't know," he said.

Neither does Mike Madrid.

California battleground

In California, it's possible no Republican will get enough votes to be on the ticket for the state's Senate seat, the first time that's ever happened. California — diverse, with a growing Latino population — reflects what much of the country is expected to look like in a couple of decades. And that, Madrid said, may doom the party, which is "teetering on the brink of the complete irrelevancy."

"That is a likely preview of coming events for the national party if it doesn't foundationally and structurally immediately adjust course," Madrid said.

Murphy agreed.

"The most dynamic vote in the country – the fastest-growing vote – is Latino-Americans. And if the Republican party does not get more competitive with that vote, and stops losing them by 30 points or more, we're never going to win a general election again."

He does point to one hopeful trend: recent polls suggest Latinos' anti-Trump views are softening. A little.

"Trump understands our perspective," said Kayla Vasquez. The Mexican-American teen was in line to see Trump speak in San Diego.

Hard to be a young Trump supporter

Being a young Trump supporter isn't easy, she said.

"When I'm in school and I want to say that I believe in Trump, they criticize me, so I just stay quiet," Vasquez said. "When you say you're voting for Trump, they say 'why are you voting for Trump? You're racist.'"

She doesn't think Trump's a racist. Her mother is a border agent. She's visited the border fence and the jails. She agrees with Trump; she says too many Mexican criminals are finding their way across the border.

"I've seen the walls that we have across the border; there's holes in them and everything," Vasquez said. "So it scares me a lot that these walls aren't sturdy enough to keep them out."

Madrid said any Republican candidate who can get a measly one in three Latinos to vote for them has a great shot at the White House.

But this year that seems unlikely.

"He doesn't need to make enormous inroads to be successful, he just needs to get above that 27-30 per cent spot," Madrid said. "But that means really doubling or tripling his current base level of support, which is a long way to go."

At the Trump rally, the chants became more racist. Some of the Trump supporters yelled "Beaners!," others yelled "Go back to Mexico!" But they always came back to their favourite: "Build! The! Wall!"

Some of the Mexican-Americans pulled bandanas over their faces. Every five minutes or so, a new fight broke out.

Police put on riot gear and moved in.

Some Trump supporters gleefully yelled "Tear gas! Tear gas!"

Soon there was tear gas.

It's about to get ugly.