But the decision to forge ahead created a wave of internal backlash, including from a top Trump adviser who said the president was playing a game of Russian roulette in holding the indoor rally. The adviser, who requested anonymity so as not to anger Mr. Trump, said the campaign was taking a cavalier approach to the pandemic that could backfire politically. Some of the president’s most vocal defenders outside of the administration agreed.

“Indoor rallies are irresponsible,” Ari Fleischer, a former White House press secretary for President George W. Bush and a frequent defender of Mr. Trump’s, wrote on Twitter. “Covid-19 is real and this was a bad idea.”

In an interview, Mr. Fleischer said he thought the president would be hurt politically if there were surges of positive cases in Las Vegas. Weeks after a rally in Tulsa, Okla., in June, the last indoor gathering the president convened, the city’s top health official said an increase in coronavirus cases in the area was probably connected to the event. Herman Cain, the former Republican presidential candidate, died of Covid-19 after attending the Tulsa rally, although it was not clear whether he was exposed to the virus there or elsewhere.