STEVE MNUCHIN, the treasury secretary, once expected to have passed tax reform by now. The Republicans’ tortuous and ultimately doomed struggle to reform health care put paid to that goal. Yet the Trump administration still promises to cut taxes in 2017, and Republicans in Congress are desperate to show that their control of the legislature is worth something. On July 27th leaders from Congress and the White House announced they had agreed on the “principles” of tax reform. Four days later, the White House set out an “aggressive” timetable to pass a bill through the House in October and the Senate in November. Yet Republicans are not as united as they seem on tax policy. Their plans will probably go the way of Mr Mnuchin’s earlier schedule—and could even meet the same fate as their health-care bill.

Tax “reform” is probably a misnomer. The last true reform, in 1986, under Ronald Reagan, reduced tax rates without losing revenue by eliminating swathes of tax deductions. Crucially, it was a bipartisan effort, which made it easier for Congress to take on the interest groups which avidly defend the benefits they gain from carve-outs. Despite apparent overtures from both sides, there is little serious hope of cross-party co-operation this time, because Democrats would extract too high a price. For example, they insist that the wealthiest 1% of Americans must not see any tax cuts—a demand that clashes with all recent comprehensive conservative plans.

Acting alone, Republicans are unlikely to broaden the tax base much. The administration has shown little appetite to take on the most expensive and distorting tax breaks: those for mortgage interest and employer-provided health insurance, and, for corporations, a deduction for debt interest. All three are defended by fierce lobbies. The only deduction that seems firmly in the administration’s sights is for state and local taxes, which mainly benefits residents of high-tax Democratic states such as California and New York. Tax-cutters rail against the corporate-tax system for favouring particular industries. But the biggest such breaks go to domestic manufacturers and oil and gas producers. President Donald Trump seems unlikely to want either scrapped. With their new principles, the Republicans’ potential tax base actually shrank, because they ruled out a controversial but lucrative “border adjustment” to the corporate tax, which would have taxed imports and subsidised exports.

That makes a tricky task trickier. To pass a tax bill by themselves, Republicans will need firm discipline. Senate procedure allows tax legislation to pass with just 51 votes, but only once a budget is in place. Passing a budget means deciding in advance whether—and to what extent—tax cuts will be paid for. If they are unfunded, the budget must waive Senate rules preventing bills from adding to deficits (even then, unfunded tax cuts could probably only last ten years). Republicans would also need to circumvent a law, signed by Barack Obama in 2010, that triggers automatic spending cuts to plug any shortfall. These would fall on programmes including Medicare, health insurance for the old, which Mr Trump has pledged to protect. Early drafts of the budget in the House of Representatives would not increase deficits. But fights have broken out over how to make the books balance. To pay for tax cuts, conservatives are proposing cuts to entitlement spending, worth $200bn over a decade, that more moderate Republicans cannot stomach. As a result, the budget is already behind schedule. It could be further delayed by a row over the debt ceiling, which must be raised in October. If so, the timetable for tax reform will slip, too. If they are constrained by a thrifty budget, Republicans in Congress will find it hard to deliver the big tax cuts they promise. Mr Trump wants all firms to pay no more than a 15% rate, to lower income taxes across the board and to eliminate the estate (inheritance) tax. Together these changes would cost over $5trn, before accounting for their effect on economic growth, according to the Tax Foundation, a right-leaning think-tank. Abolishing the state and local deduction raises only $1.8trn. A little more money—perhaps $260bn—might come from taxing profits that firms have stashed overseas. Yet this would be a one-off windfall that could finance only a two- or three-percentage-point reduction in the corporate rate for a decade. Congress could square the circle by directing budget scorekeepers to assume that tax cuts spark much higher economic growth. But although most congressional Republicans support such “dynamic scoring”, they have little appetite for completely fanciful analysis, says Andy Laperriere of Cornerstone Macro, a consultancy. A credible dynamic score would still leave a large funding gap.

The upshot is that modest, permanent tax cuts, financed by scrapping the state and local deduction, are achievable. In isolation, such a reform could even fulfil Mr Mnuchin’s pledge not to cut income tax for the highest earners (who also pay the most to state and local governments). Such a policy might be combined with temporary business-tax cuts that balloon the deficit and benefit the rich. But that would require advanced planning in the budget and a careful legislative strategy. Firms should not bank on it.