Republicans have a math problem.

The tax reform bill they are pushing through the Senate will live and die by a complicated rule — known as the “Byrd Rule,” a condition of the “budget reconciliation” process that allows Republicans to pass legislation with only 51 votes in the Senate. Because of how they set it up, Republicans’ tax bill can only increase the deficit by $1.5 trillion in the first 10 years, with no increase outside that window.

But as it stands, neither the House tax bill nor the Senate’s passes this test.

The Congressional Budget Office estimates the House bill will increase the deficit by $1.7 trillion, and the initial analyses of the Senate bill show it is on track to increase the deficit outside the 10-year window — the plan’s largest impact on the deficit is by $216.7 billion in 2027.

Maneuvering around Senate rules is not new to congressional Republicans, who recently tried, and failed, to pass Obamacare repeal through budget reconciliation. They said tax reform would be easier. Both the House and Senate bills have tried to repeal or cap various deductions — some big, some small — earning the ire of various factions of the party. But they haven’t yet gotten the math to work.

It will all come down to what the Senate parliamentarian will permit. There’s no question that Republicans are desperately in search of budget gimmicks and rosy economic projections to make it all work — the question is if it will.

The Byrd Rule is the price of partisan politics

At the beginning of this year, with a bare 52-vote majority, far short of the 60-vote filibuster threshold, Republican leadership devised a plan to bypass Democrats altogether on major legislation: They would tie their major agenda items to the budget through “budget reconciliation,” a bill that can impact spending, revenue, or the debt ceiling with only a party-line vote in the Senate.

It’s a process President Bill Clinton used to pass welfare reform in 1996 and President George W. Bush used to pass tax cuts in 2001 and 2003. It’s how President Barack Obama got several budgetary amendments to the Affordable Care Act passed. Republicans also attempted to use budget reconciliation to try to pass an Obamacare repeal bill in the Senate

As Vox’s Dylan Scott explained, any bill being passed under reconciliation has to comply with every section of the six-part Byrd Rule. “If it fails any one of those tests, it must be stripped out”:

1. The provision must change federal spending or revenue. 2. If the bill does not meet the budget resolution’s instructions to reduce the federal deficit, any provision that results in either increased spending or decreased revenue is removed until it does meet those targets. 3. The provision must only affect policies that fall under the jurisdiction of the specific committees that were instructed in the budget resolution. 4, The provision’s effect on spending or revenues must be more than incidental to its policy impact. 5. The provision cannot increase the federal deficit at some point in the future, beyond the typical 10-year “budget window” that is used to evaluate legislation. 6. The provision cannot change Social Security.

Republicans have tied their hands: They have passed a budget that allows them to increase the deficit by $1.5 trillion in the first 10 years, but they want to pursue massive tax cuts that appear to increase the deficit outside that 10-year window.

House Republicans are expected to vote on their tax bill this week, as the Senate is marking up its version — both would have to see substantial changes to pass the Byrd test.

The ways Republicans are trying to get around the Byrd question

Already Republicans are using gimmicks to get around this, the major one being passing what appears to a sweeping tax cut that will sunset after 10 years. This is what former President George W. Bush did with his tax cuts under budget reconciliation in 2001, essentially getting around the deficit restriction by making the tax cuts temporary. This House included some of this, sunsetting the $300 family credit after five years. The Senate could do the same.

There are already rumors of a temporary and permanent split in tax reform proposals among Republican members. The question is whose tax cuts are permanent and whose are temporary. Sen. Orrin Hatch (R-UT), who chairs the Senate Finance Committee, said his committee has "every intention of making the business reforms permanent" at the start of the bill’s markup — meaning it might be the individual tax cuts that will sunset, instead of the corporate rate.

One way this happens, floated by Zach Moller, a senior policy analyst for the Committee for a Responsible Federal Budget, is what he calls the “Title Gambit.” Republicans could split up their tax bill into three separate titles: One would be a temporary Republican-led effort that would pursue aggressive tax cuts; a second would be permanent and comply with Senate rules; and the third would be a permanent bipartisan proposal needing 60 votes that would likely find consensus around issues like the child tax credit or doubling the standard deduction. By going this route, Republicans would be able to split the impact of the deficit between budget reconciliation bills and regular order.

But again, it comes down to whether the parliamentarian will go for these separate titles, which are usually separated by committee involvement.

Some analysts have also floated the nuclear option, in which the presiding officer of the Senate — Vice President Mike Pence — just overrules the Senate parliamentarian. McConnell did not allow for this in the health care debate, but some have theorized there is more pressure for him to go to extreme lengths this time.

Either way, Republicans are relying on projections of increased economic growth from tax cuts to offset the revenue losses from those cuts, known as “dynamic scoring,” to sell their tax bill.

The Senate Budget Committee could also use a different, more ideologically conservative score of their tax plan instead of the CBO’s evaluation, like the Tax Foundation’s score or the Treasury Department’s.

But currently, even under the rosiest of projections, economic growth alone doesn’t seem to be enough to offset the full losses from the deepest cuts Republicans have proposed. The conservative Tax Foundation found the House tax bill would still increase the deficit by $500 billion, and projections from the University of Pennsylvania’s Wharton School showed that by 2040, the debt would increase by $6.4 trillion to $6.9 trillion.

The tax cuts aren’t paying for themselves. And there’s still skepticism whether the Senate parliamentarian will accept those growth numbers.