Mr. Scott said that he had discussed the plan with Mr. Trump and that the president had later spoken approvingly of it. But in the rush to pass the bill over the course of a few frenzied weeks, the idea was never debated on the floor of the House or Senate. It was never promoted by Republican leaders or the White House.

“This is a little billion-and-a-half dollar part” of the law, Kevin Hassett, the chairman of Mr. Trump’s Council of Economic Advisers, said in an interview. “But if it’s successful, we’ll look back 10 years from now and say this was one of the most important parts of the tax bill, and one we didn’t talk nearly enough about.”

Mr. Hassett has a longtime interest in providing tax incentives for economic development in distressed areas. He said he first began discussing opportunity zones with Mr. Parker several years ago at a meeting in Mr. Parker’s Greenwich Village home. Before joining the Trump administration, Mr. Hassett wrote several white papers to help elevate the idea as part of an extensive, multiyear effort by the Economic Innovation Group to win support.

Mr. Hassett said he was never paid for any of that work. His interest, he said, stems from growing up near Turners Falls, Mass., which has struggled since the closing of its longtime paper mills.

Mr. Parker, who made his fortune as the first president of Facebook, was looking for a way to steer investors to parts of America that have been starved for economic activity in the wake of the Great Recession.

One in six Americans lives in what the Economic Innovation Group calls a “distressed community,” where median household incomes remain far below the national level, which is $59,000 a year, and the poverty rate is well above the national average. Those communities are urban, rural and suburban. On average, the communities lost 6 percent of their jobs and a similar share of their business establishments from 2011 to 2015, according to census data.