Republicans for Hillary co-founder Ricardo Reyes in Washington DC. AP Donald Trump's business background is one of his main selling points, but that hasn't been enough to endear him among many Silicon Valley business leaders.

The Republican presidential nominee is running a pessimistic campaign, whereas the default outlook of Silicon Valley is optimism, according to Ricardo Reyes, a tech industry insider who previously headed communications for Tesla, Google's YouTube, and Square.

Reyes is not some techie progressive. A longtime Republican, Reyes served in the Bush administration.

Now he's one of the cofounders of Republicans for Clinton, a self-described grassroots network of Republican voters who are actively supporting Democratic presidential nominee Hillary Clinton.

Silicon Valley "is a very accepting culture, you have different people from different walks of life, you have different people from different cultures all kind of working together. It really is a melting pot of cultures here. And that kind of rejects that whole view he has," Reyes told Business Insider.

The fact that high-profile Silicon Valley Republicans like Reyes are actively opposing Trump highlights a big challenge facing the candidate as he tries to broaden his appeal to voters. While tech investor Peter Thiel endorsed Trump at the Republican National Convention, outward signs of support for Trump in the Valley are otherwise tough to find (one tech worker Business Insider spoke to said that he had to hide his support for Trump because the candidate is so unpopular in Silicon Valley).

"Let's talk about the make-up of Silicon Valley. Let's think about who actually comes out to CA and works out here. These are typically very risk tolerant individuals. They believe in opportunity. They actually don't believe this whole 'Make America Great Again' thing," Reyes says.

Trump's isolationist and anti-free trade stances might also be scaring the big tech companies. "What you have here is a bunch of companies that think at a very global scale and when you break down his arguments on isolationism and how we need to pull back from our international partnerships, that just doesn't fly here," Reyes said.

Notable tech company execs including Alphabet Executive Chairman Eric Schmidt, Facebook COO Sheryl Sandberg, and Apple CEO Tim Cook have all endorsed Clinton.

Republicans for Clinton

Republican presidential candidate Donald Trump speaks to an overflow crowd during a campaign rally, Tuesday, Sept. 6, 2016, in Greenville, N.C. Evan Vucci/AP

Before Reyes became a Silicon Valley crisis communicator, he worked in the Bush administration, in a position advising on trade communications. Before that, he edited Regulation Magazine, a Cato Institute publication.

When he started working in tech, he thought he had left Washington DC behind. But the "threat of a Trump presidency" was so egregious to him that he had to get involved — on behalf of Clinton. "When I left Washington I dedicated myself to the private sector," Reyes said.

Republicans for Clinton isn't a big PAC. In fact, it only recently started taking donations. Instead, Reyes says the strategy is to hold events in swing states, especially to help Republicans down the ballot, and place editorials in influential publications.

"What we're trying to ensure is that Trump is defeated, that Hillary wins the presidential election, but also that good, down-ballot Republicans don't suffer too much at the ballot box because of Trump's unpopularity," Reyes says.

Recently, Republicans for Clinton co-founder John Stubbs placed an editorial in the Washington Post, for example. Republicans for Clinton also supports Jim Glassman, a Clinton endorser and American Enterprise Institute fellow who goes on TV often, and other notable Republicans. Reyes says his organization is in "loose communication" with the official Clinton campaign, mostly through third-parties.

Reyes thinks that Bush-era Republicans and Silicon Valley libertarians have a lot in common — although he's not supporting Libertarian candidate Gary Johnson, who he says has a "progressive mindset."

"People should be trusted to make the best decisions for themselves. That was a very strong Republican tenant but it is also a very strong technology tenant," Reyes said. "We believe in individual responsibility, individual liberty, individual freedom. Sadly, none of the things that Donald Trump seems to believe."