Both billionaire donors are close to Mr. Trump, a fellow tycoon. Mr. Adelson played a pivotal role in Mr. Trump’s election, showering Republican groups last year with tens of millions of dollars. Mr. Wynn is the finance chairman of the Republican National Committee and oversaw a fund-raiser on Wednesday at the president’s Washington hotel that Mr. Trump said had raised about $7 million for the party committee and his re-election campaign.

Earlier that day, America First Policies held a donor meeting for those who were in the capital for that evening’s fund-raiser. Every contributor who raised the issue of the anti-Heller campaign — an extraordinary offensive against a vulnerable senator in one’s own party — expressed approval of the attacks, according to an attendee.

Ronald M. Cameron, a major Republican donor who gave the maximum $5,400 donation to Mr. Heller’s re-election campaign this year, said he would consider investing in primary race challenges to Republican lawmakers who oppose the health care bill or other White House legislative priorities.

“I might support a challenger, and would certainly withhold support from someone that I thought was against Trump’s agenda,” said Mr. Cameron, an Arkansas poultry magnate who donated more than $2 million to committees supporting Mr. Trump’s 2016 campaign and attended the Wednesday fund-raiser for his re-election.

Mr. Cameron — who was solicited by America First but said he had not donated to the group — said that he was not familiar with the group’s ads against Mr. Heller, but that he did not object to the idea of publicly calling out lawmakers who oppose the health care bill.

They should “shape up or get out of the way,” he said.

Mr. Trump himself, while acknowledging the complaints of the Republican senators at the White House meeting, has in other private sessions with his aides and allies made clear that he very much approved of the onslaught against Mr. Heller. At the wedding of Treasury Secretary Steven Mnuchin last Saturday and then again in the White House this week, he told advisers that he supported the ad blitz, according to multiple Republican officials who have spoken to the president.