The tax overhaul the Senate approved on Saturday rests on a shaky stack of promises Republicans will be hard-pressed to keep. Foremost are the two central assurances GOP senators made to justify passage of the bill, which may be irreconcilable with each other. First, Republicans have insisted that the measure’s $1.4 trillion cost would disappear under a bustle of economic activity—more jobs, higher wages, and buckets full of new tax revenue flowing into the government. “The plan will pay for itself with growth,” Treasury Secretary Steven Mnuchin declared this spring, a claim that he and other administration officials and Republicans repeated throughout the summer and fall. They made this assertion despite all available evidence to the contrary—the past experiences of the federal tax cuts enacted under George W. Bush and the more recent failure of tax cuts in Kansas; independent analyses by the Tax Foundation, the Tax Policy Center, and other outside arbiters; and, most flagrantly, by Congress’s own in-house analysts at the Joint Committee on Taxation, who found that the Senate bill would provide a tepid boost to economic growth and would recoup barely one-third of its cost over 10 years. Republicans are also promising that millions of families will never see the future tax increases that they wrote into the bill to make it work under the Senate’s budget rules. The expiration in six years of most of the benefits for individuals is a fuse that will never be lit, the GOP insists, under the presumption that lawmakers would never abide a tax increase that large. Yet if that fiscal bomb never goes off, the bill’s true cost would soar even higher still, making the party’s first promise, that the bill would pay for itself, even more unattainable. In addition to the major pledges they made to the public, GOP leaders made more discreet, shorter-term promises to round up the final votes for the bill, on issues far afield from tax policy. Those will be equally tough to keep, and a failure to follow through could jeopardize the tax legislation’s final enactment after negotiations in a conference committee of the Senate and House. To win over Senator Susan Collins of Maine,