WASHINGTON — The White House plan for a massive package of tax cuts is gaining new momentum as Republicans try to set aside months of intraparty squabbling and unify behind a key part of President Trump’s agenda.

Two developments are accelerating the effort: Key Senate Republicans reached a tentative deal this week to allow for as much as $1.5 trillion in tax reductions over 10 years; and there is a growing willingness within the party to embrace controversial, optimistic estimates of how much economic growth their tax plan would create.

Those upbeat estimates, often rejected by bipartisan economists, would supplant the traditional forecasts offered by official scorekeepers at the Congressional Budget Office and Joint Committee on Taxation, helping lawmakers argue that the plan would not increase the national debt.

Trump is pushing for what he says will be the largest tax cut in U.S. history, which advisers say will come from a sharp decrease in corporate tax rates and tax relief for the middle class.

Numerous pitfalls remain, and Republicans have not yet agreed on major aspects of the plan. They haven’t reached a deal on what the tax cut’s impact should be on the budget deficit, what tax breaks should be jettisoned, or whether to pursue permanent tax cuts or ones that would expire after a number of years. Meanwhile, House conservatives continue to threaten to block any deal unless the White House agrees to include large spending cuts in any tax package. Fights over any of these issues could derail the discussions.

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