The Republican tax reform plan is suffering from a shortage of funds. Paul Ryan and his colleagues are trying to give the American people a substantial tax cut as their signature achievement of 2017.

However, they are constrained by Congressional rules that limit increases in the federal budget deficit to just $1.4 trillion over 10 years. As a result, the plan is peppered with small-bore cutbacks to tax deductions and other maneuvers that make the plan seem small-minded and miserly.

Thankfully, there is one potentially large tax reform that could bring in a whopping $1 trillion, and empower the Ways and Means Committee to be truly generous with the American middle class.

There is a move toward sales factor apportionment (SFA) on the corporate income tax. The current Republican plan (also known as TCJA—the Tax Cuts and Jobs Act) sounds bold because it is slashing the top corporate income tax rate from 35 percent to 20 percent. Yet aside from that change on the corporate side, TCJA is basically a nip and tuck job. It fixes a few loopholes, but keeps most corporate loopholes, deductions, and exclusions in place—leaving us with one of the leakiest corporate tax systems in the world. Billions of dollars in corporate profit will continue to escape the IRS because they will be hidden in offshore subsidiaries or other tax structures that exempt funds from US taxation. This is called base erosion, since the tax base of corporate profits earned by US-based corporations escapes the IRS through clever tax planning.

SFA corporate income tax would be truly fundamental reform. SFA taxes corporations based on the portion of their profit that corresponds to their US sales. If they make half of their sales in the US market, then they pay tax on half of their global profits; it doesn’t matter if they use clever accounting to attribute profit to an offshore tax haven; it doesn’t matter if the consumer buys the product from a subsidiary in a tax haven. The data the IRS needs to calculate tax liability under SFA is much more readily available and much harder to fudge than the data used under the present system.

In an article to be published later this month in Tax Notes, we estimate that a move to SFA taxation would raise US corporate income tax revenue by a whopping 34 percent a year. That 34-percent uplift comes mainly because there are so many large corporations paying far below headline tax rates today. Just by requiring them to pay the headline rate on their US-destination profits raises the tax take substantially.

Here are the numbers: According to estimates from the Congressional Budget Office, the current tax system is set to generate $3.9 trillion in corporate income tax revenue over the 10 years from 2018-2027. Corporate tax cuts in the TCJA (mainly the rate cut from 35 percent to 20 percent) slice $847 billion off that total. A switch to SFA taxation, while maintaining the 20 percent rate, would add an estimated $1.04 trillion to the 10-year tax take. In 2018 alone, a move to SFA would add $68 billion to the corporate tax take.

That extra trillion dollars would make many other tax cuts feasible, in particular on the individual tax side. For example, the current plan to eliminate the deductibility of state and local tax payments could be dropped; the cap on mortgage interest deductibility could be raised or eliminated; and, most fundamentally, income tax rates for the middle class could be lowered further, making the tax package unarguably a “middle class tax cut.”

The Republicans claim they are attacking the erosion of the corporate tax base with their proposed 20 percent excise tax on payments by US corporations to foreign-based affiliates. But only days after lobbyists found out about the excise tax, the Republicans have reportedly added new exceptions that reduce its revenue by as much as 95 percent. This only makes a complex, inefficient system even more complex. It’s a sure bet that, within a short space of time, clever corporate accountants will find new ways to avoid what remains of the excise tax. In fact, you could almost rename the corporate side of this act the Tax Cuts and Jobs for Accountants Act.

There are other advantages to SFA taxation, such as greater fairness, greater predictability of future revenue, and benefits for small companies. So why haven’t the Republicans embraced it? The answer is that too many huge corporations paying little or no tax have been lobbying behind the scenes for months to prevent fundamental reform of today’s corporate tax system. It’s time for Congress to face up to the vested interests and consider some fundamental corporate tax reform.

Jeff Ferry is research director at the Coalition for a Prosperous America.