A powerful array of the Republican Party’s largest financial backers remain deeply resistant to Donald Trump’s presidential candidacy, forming a wall of opposition that could make it exceedingly difficult for him to meet his goal of raising $1 billion before the November election.

Interviews and e-mails with more than 50 of the party’s largest donors, or their representatives, revealed a measure of contempt and distrust toward their own party’s nominee that is unheard of in modern presidential politics.

More than a dozen of the party’s most reliable individual contributors and wealthy families indicated that they would not give to or raise money for Trump.

So far, Trump has embraced the hostility of the GOP establishment, goading the party’s angry base with diatribes against wealthy donors who he claimed controlled politicians. He has funded his operation with a mix of his own money and small-dollar contributions.

But that formula will be tested as he presents himself to a far larger audience of voters. Trump has turned to the task of winning over elites he once attacked, with some initial success. And he has said that he hopes to raise $1 billion, an enormous task given that he named a finance chairman only this month.

Among the party’s biggest financiers disavowing Trump are Paul E. Singer, a New York investor who has spent at least $28 million for national Republicans since the 2012 election, and Joe Ricketts, the TD Ameritrade founder who with his wife, Marlene, has spent nearly $30 million over the same period of time, as well as hedge fund managers William Oberndorf and Seth Klarman. “If it is Trump vs. Clinton,” Oberndorf said, “I will be voting for Hillary.”

The rejection of Trump among some of the party’s biggest donors and fundraisers reflects several strains of hostility to his campaign. Donors cited his fickleness on policy and an ad hoc populist platform focused on trade protectionism and immigration.

Among the more than 50 donors contacted, only nine have said unambiguously that they will contribute to Trump. They include Sheldon G. Adelson, the casino billionaire; energy executive T. Boone Pickens; and Foster Friess, a wealthy mutual fund investor. Friess wrote in an e-mail that Trump deserved credit for inspiring “truckers, farmers, welders, hospitality workers — the people who really make our country function.”

Many more donors declined to reveal their intentions or did not respond to requests for comment, a remarkable silence about the de facto nominee of their party.

Asked how Trump intended to win over major donors, Hope Hicks, a spokeswoman for the Trump campaign, said, “There is tremendous support for Mr. Trump.”

Trump has declared that he expects the Republican Party to unite around him, and in recent weeks has made inroads among party leaders. Polls show the party’s rank and file are beginning to coalesce behind him.

Some major donors have not explicitly closed the door on helping Trump but have set a high bar for him to earn their support.

“Until we have a better reason to embrace and support the top of the ticket, and see an agenda that is truly an opportunity agenda, then we have lots of other options in which to invest and spend our time helping,” said Betsy DeVos, a Michigan Republican whose family has given nearly $9.5 million over the last three elections to party causes and candidates.

But others simply believe Trump is unfit. Michael K. Vlock, a Connecticut investor who has given nearly $5 million to Republicans at the federal level since 2014, said he considered Trump a dangerous person. “He’s an ignorant, amoral, dishonest and manipulative, misogynistic, philandering, hyper-litigious, isolationist, protectionist blowhard,” Vlock said.

Unless Trump can win over more benefactors, he is likely to become the first Republican presidential nominee in decades to be heavily outspent by his Democratic opponent. Both President Obama and Mitt Romney raised more than $1 billion in 2012, and Clinton is expected to exceed that figure easily.