Mr. Trump, who by the end of March had spent around $40 million of his fortune on the primaries, has said that he may need as much as $1.5 billion for the fall campaign, but that he will seek to raise it from donors rather than continue to self-finance.

But Mr. Trump has no fund-raising apparatus to resort to, no network of prolific bundlers to call upon, and little known experience with the type of marathon, one-on-one serial salesmanship and solicitousness that raising so much money is likely to require — even if individuals can contribute up to the current limit of $334,000 at a time to the party. And he has to do it all in six months, with a deeply divided party that is still absorbing the fact that Mr. Trump is its standard-bearer.

“No one should underestimate how hard it would be for any nominee to raise hundreds of millions of dollars in a very short period of time,” said Mike DuHaime, who was the top strategist for the presidential campaign of Gov. Chris Christie of New Jersey…

Underscoring the urgency with which Mr. Trump and Republicans will need to increase their fund-raising, some of the party’s allies who spent enormous sums in the 2012 election now appear likely to stay on the sidelines in the presidential race — including the vast Koch brothers network, which had pledged to spend nearly $900 million in 2016.