When Republicans in the House and Senate unveiled their tax bills to great fanfare over the last two weeks, they glossed over a small but critical detail: Neither of them, as written, can pass Congress with GOP votes alone. Both proposals are over-budget, analysts say, and would require significant revisions to abide by Senate procedural rules. Republicans likely will adjust their plan by making some of the biggest tax cuts expire in the next decade, a change that limits the legislation’s potential for economic growth and would force lawmakers to confront potential tax increases years in the future. To circumvent a Democratic filibuster in the Senate, Republicans are following the same budget reconciliation rules they used when they tried to repeal the Affordable Care Act earlier this year with a simple, 51-vote majority. Under parameters set by the congressional budget resolution adopted last month, the tax bill can add up to $1.5 trillion to the deficit in the first decade after its enactment, but it cannot add anything to the deficit in the years following that. The rule is named for the late Senator Robert Byrd of West Virginia, the long-serving Democrat known for his parliamentary expertise. The proposal House Republicans approved in the Ways and Means Committee last week meets the first test but not the second: Steep cuts to the corporate and individual tax rates would cost more money than the government would bring in through the elimination of popular deductions and exemptions, even when accounting for economic growth. The Senate “has an even bigger problem,” said Ed Lorenzen, a senior adviser for the Committee for a Responsible Federal Budget. The bill released on Thursday differs from the House proposal in a number of respects. While it completely eliminates the deduction for state and local taxes, it maintains the mortgage interest tax deduction and a few other expensive tax breaks. The Senate bill also reduces the income tax rate paid by the wealthiest earners, while the House bill does not. “The Senate bill on a permanent ongoing basis has a much larger cost, a much larger deficit than the House bill,” Lorenzen told me. Both House and Senate