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, top Republicans committed to passing a pair of bipartisan health-care bills aimed at bolstering the Affordable Care Act’s faltering individual-insurance market. Collins demanded those bills to mitigate the impact of repealing the law’s individual mandate, which she supported as part of the tax plan despite voting against several earlier GOP proposals to do so. One bill would restore the payments to insurers that President Trump canceled earlier this fall, while another written by Collins and Democratic Senator Bill Nelson of Florida would fund reinsurance programs to offset the cost of covering the most expensive patients.

Collins had said she wants to see those measures signed into law before enactment of the tax bill, which would give her leverage in a final vote on the conference report. But that presumes passage by the House, where conservatives are warning their leadership against any legislation that would prop up Obamacare. In a statement announcing her support for the tax bill, Collins said only that Majority Leader Mitch McConnell had promised his support for passing those bills by the end of the year—leaving open the possibility that her vote for the tax overhaul requires a leap of faith on health care. If the health bills stall, would she reverse her vote on tax cuts?

Similarly, Senator Jeff Flake of Arizona said he secured a commitment from both the Senate leadership and the Trump administration on immigration in exchange for his vote. They agreed, he said in a statement, “to work with me on a growth-oriented legislative solution to enact fair and permanent protections for DACA recipients.” In fairness to Flake, he’s not quite saying Republicans promised to pass such a bill—only to help him try. But it’s still a curious commitment on which to base his vote considering that Trump has been escalating his rhetoric against illegal immigrants of late and accusing Democrats allied with Flake on the issue of wanting undocumented immigrants “flooding into the country.” Moreover, with Democrats and even some Republicans insisting that a DACA fix be attached to a year-end spending bill, the president is reportedly girding for a government shutdown to rally his base.