WASHINGTON — The Republican tax plan promoted by President Trump this week as a middle-class tax cut would overwhelmingly benefit the wealthiest Americans and businesses, according to an analysis released on Friday by the nonpartisan Tax Policy Center.

The report, which is the first detailed assessment of the plan’s financial impact, found that the average tax bill for all income groups would decline by $1,600, or 2.1 percent, in 2018. The biggest decrease would go to those with incomes above $730,000, who would see their after-tax incomes rise by an average of 8.5 percent, or about $129,000.

Those in the middle quintile — with incomes averaging $66,960 — would see their after-tax income rise by 1.2 percent or about $660.

The breakdown is based on the framework released by the “Big Six” group of Republican lawmakers and administration officials this week, which did not include many details that could change the distributional impact. For instance, the plan called for an increase in the child tax credit but did not specify how much it would rise and whether it would be across income groups. The plan also opened the door for adding a fourth, higher tax bracket for the richest Americans, which would also change the distributional impact if enacted.