The gap between the rich and the poor in the Democratic presidential primary is growing — and Beto O’Rourke has landed on the worrisome side of the dividing line.

Mr. O’Rourke, who had entered the race in March with the promise of being a top candidate and a financial powerhouse, faltered in dramatic fashion in second-quarter fund-raising, announcing on Monday that he had taken in only $3.6 million. The sum was a fraction of what the front-runners brought in.

In fact, Mr. O’Rourke’s total for the 90-day period was far less than he raised in his first 24 hours as a candidate, when he received $6.1 million.

“We had a lot of that energy early,’’ Jennifer O’Malley Dillon, Mr. O’Rourke’s campaign manager, said in an interview Monday night. “We didn’t have the organization that could capitalize on that. And that’s what we’ve been building.’’