Many Republicans have argued that economic growth will either largely or fully cancel out the revenue lost by trimming the tax burden on corporations and individuals. GOP lawmakers like outgoing Sens. Jeff Flake, R-Ariz., and Bob Corker, R-Tenn., have expressed concerns about a bill significantly increasing budget deficits.
The Penn Wharton model estimates the Senate Republican plan will fall short of paying for itself.
The Senate bill as amended last week would temporarily cut many individual taxes, while permanently chopping the corporate tax rate to 20 percent from 35 percent. Individual changes expire to help the plan to comply with Senate budget rules.
While the GOP hopes to eventually extend individual tax cuts, the Penn analysis assumes they expire before 2027, as written.
The Senate proposal would cause GDP to rise 0.3 percent to 0.8 percent in 2027 relative to current law, the report estimates. It would be 0.2 percent to 1.2 percent higher by 2040, it adds.
In 2040, U.S. revenues would fall by $1.1 trillion to $2.1 trillion, while debt would climb by $1.7 trillion to $2.4 trillion, the Penn model projects.
The congressional scorekeeper, the Joint Committee on Taxation, has separately estimated that the Senate bill would increase deficits by $1.4 trillion over a decade.
Last week, the House passed a similar tax bill, which would not make individual tax changes expire. The Senate hopes to pass its version next week.
Once both chambers pass a bill, they have to reconcile the proposals before they can approve final legislation to send to President Donald Trump.
Republicans hope to pass a tax bill before the end of the year.