The Joint Committee on Taxation said Wednesday afternoon that the Senate tax bill would add $1 trillion to federal budget deficits over the next decade, even after accounting for additional economic growth, a major blow to Republicans’ contention that the $1.4 trillion tax overhaul would pay for itself through growth.

The committee, which serves as the nonpartisan scorekeeper for growth and revenue estimates in tax bills, estimated that the Senate bill would boost economic growth by 0.8 percent over a decade. Republicans have said they expect substantially stronger growth than that to result from the tax cuts.

Throughout the tax debate over the last month, Republican leaders have frequently cited other analyses by the committee in order to make their case for the bill.

The committee said economic growth generated by the tax cut will offset losses by about $458 billion over the next decade. Over that same period, an additional $51 billion would be needed for interest costs.

“Overall, the budgetary effects of changes in economic growth are projected to reduce the deficit by $407 billion during the budget window,” the committee said.

Republican leaders threw cold water on the analysis.

A spokeswoman for Senator Orrin G. Hatch, the Utah Republican who leads the Finance Committee, said the joint committee’s analysis understates the actual economic growth and is not based on the current version of the bill.

“An analysis of tax provisions that do not reflect the final outcome of the evolving Senate tax bill — which will be amended on the floor this week — is incomplete,” said the spokeswoman, Julia Lawless. “The nonpartisan Congressional Budget Office (C.B.O.) has said it was ‘not practicable’ to issue a macro view of the Senate bill at this time. And given that leading economists have projected the Senate tax bill will deliver significantly higher amounts of economic growth and federal revenue than the Joint Committee on Taxation (J.C.T.) reports, the findings of J.C.T. are curious and deserve further scrutiny.”