Mr. Trump is already raising far less money directly for his campaign than Mrs. Clinton is raising for hers, taking in $42 million in August compared with her $59 million. Now he is also heading toward Nov. 8 with relatively limited financial help from super PACs and outside groups that can accept unlimited contributions from rich donors.

Groups supporting Mr. Trump have aired just $12 million in broadcast advertising, according to the Campaign Media Analysis Group. Mrs. Clinton’s allies, buoyed by a sharp increase in giving by wealthy liberals on Wall Street and in Silicon Valley, are expected to spend at least $160 million by Election Day.

“If you want to help that Senate candidate or that gubernatorial candidate, the best way we could have done it was to nominate any of the other people who were running,” said Douglas Heye, a former adviser to House Republicans. “That’s not the reality of the world right now. The best way to do that now is to help that campaign directly.”

Two Republicans with knowledge of his giving said that Mr. Adelson was allocating a far smaller sum, $5 million, to benefit the top of the Republican ticket: token support by Mr. Adelson’s standards. And in a striking move, the money will go not to any of the pro-Trump super PACs but to organizations controlled by a fellow billionaire, Joe Ricketts, a Wyoming investor, whose own political operation will decide how to spend it. The Ricketts family spent millions of dollars during the primaries to defeat Mr. Trump, who responded with a Twitter message that the Rickettses “better be careful, they have a lot to hide!”

A spokesman for Mr. Adelson declined to comment.

“Everyone reached out to him. He promised $100 million,” said Ed Rollins, an adviser to Great America, one of several outside groups supporting Mr. Trump. “At this point in time, he’s like everyone else. Now he’s going to be a player, just not to the same extent.”