Peter Thiel, co-founder of PayPal and Palantir Technologies Inc., is a well-known Donald Trump supporter. He signed on as a delegate and endorsed the presidential candidate in a speech at the Republican National Convention. He's also planning to donate $1.25 million to Trump's campaign, according to a person familiar with the matter.

What's less known is why Thiel has waited so long to donate to Trump and why he gave less than he did to Carly Fiorina’s presidential campaign last year. Thiel was probably asked to give money, and Trump wasn't his first—or, perhaps, even his second—choice, political experts say.

"It's pretty late for a major contribution to come in and be used efficiently," said Eric Schickler, a professor of political science at the University of California at Berkeley. He said the donation may be more of a symbol, albeit a pricey one, of Thiel's support for the Republican party and his desire to put one of their own in the White House. While the sum is fairly large, it falls short of the $2 million Thiel donated to a super-PAC supporting Fiorina when she declared her intent to run. This, too, isn't an accident, according to Schickler.

"He answered the question of, 'Where do you go with your support if it's not your ideal Republican?' The answer is not all the way," said Schickler. He noted that Sheldon Adelson, a casino magnate, and other normally generous Republican donors have not given large sums to Trump. For Thiel, "Trump may have been below his second choice," Schickler said.

Thiel, a pro-marijuana, gay billionaire who is on the board of Facebook Inc., has been a steady political donor to Republican and conservative groups since at least 2000, according to data compiled by the Center for Responsive Politics. Although his Fiorina donation was the largest, his Trump contribution is by far his most controversial and reverses an earlier position of not contributing money. Thiel declined to comment through a spokesman. His donation to Trump was reported earlier by the New York Times.

Silicon Valley entrepreneurs and investors, many of whom Thiel does business with, have been joining the anti-Trump movement by donating both money and mind share to thwarting his campaign. Trump's position on limiting immigration—more than half of startups valued at $1 billion or more had at least one foreign-born founder, according to a report by the National Foundation for American Policy—and recently leaked video footage of him making lewd comments about women, galvanized an already left-leaning constituency and convinced some longtime Republicans to abandon the candidate.

The leadership at Y Combinator, a startup incubator where Thiel is a part-time partner, said while they disagreed with Thiel's decision, they supported his right to back whichever candidate he chooses. "If our best ideas are to stop talking to or fire anyone who disagrees with us, we’ll be facing this whole situation again in 2020," Sam Altman, president of Y Combinator, wrote in a blog post.

Ellen Pao, a former Kleiner Perkins Caufield & Byers partner who co-founded the diversity initiative Project Include, disagreed. She said the group would sever all ties with Thiel and any groups still associated with him, including Y Combinator. "We agree that people shouldn't be fired for their political views, but this isn't a disagreement on tax policy," Pao wrote in a blog post. "This is advocating hatred and violence."

While it's unlikely that a $1.25 million donation will swing the election—Hillary Clinton has raised more than $516 million, more than double Trump's take, according to the Center for Responsive Politics—it should endear Thiel to Trump in the campaign's final days.

"Thiel was probably asked to do it," said Bruce Cain, a professor of political science at Stanford University. Cain said that with some Trump donors recently withdrawing their support and Trump seeking to limit his personal expenditures on the campaign, the fundraising drive has dialed up a few notches to campaign in Florida, North Carolina and Ohio.