A powerful array of the Republican Party’s largest financial backers remains deeply resistant to Donald J. 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 emails with more than 50 of the Republican 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 Mr. Trump. This group has contributed a combined $90 million to conservative candidates and causes in the last three federal elections, mainly to “super PACs” dedicated to electing Republican candidates.

Up to this point, Mr. Trump has embraced the hostility of the Republican establishment, goading the party’s angry base with diatribes against wealthy donors who he claimed controlled politicians. And he has succeeded while defying conventions of presidential campaigning, relying on media attention and large rallies to fire up supporters, and funding his operation with a mix of his own money and small-dollar contributions.